We'll want to eventually distinguish between sqlmigrations (only run at
start up) and general purpose (and possibly long-running) migrations.
We'll introduce the latter in future commits, within a new
pkg/migration.

Release note: None

This callback will be called after validating a `SET CLUSTER SETTING
version` but before executing it. It will be used in future commits to
execute long-running migrations.

Release note: None

This request will be fleshed out and used in future commits that
introduce the long running migration orchestrator process.

Release note: None

...and populate it with the `AckClusterVersion` op.

EveryNode is the RPC that will powers the long running migrations
infrastructure. It'll let callers define and execute arbitrary commands
across every node in the cluster. To motivate what this would look like,
we introduce alongside it one such command, the `AckClusterVersion`
operation. It isn't currently hooked up to anything, but this will
eventually be the mechanism through which we'll propagate cluster
version bumps across the crdb cluster, replacing our gossip based
distribution mechanism in-place today. This will let the orchestrator
bump version gates in a controlled fashion across the cluster.

To achieve this, the stubbed out implementation of AckClusterVersion
makes use of the same `StoreClusterVersionKey` otherwise used in
callbacks attached to the gossip handler for cluster version bumps.

Release note: None

Now that we always create a liveness record on start up (cockroachdb#53805), we can
simply fetch all liveness records from KV when wanting an up-to-date
view of all nodes in the cluster. We add a helper to do as much,
which we'll rely on in future commits. It's a bit unfortunate that we're
further adding on to the NodeLiveness API without changing the
underlying look-aside caching structure, but the methods fetching
records from KV is the world we're hoping to start moving towards over
time.

Release note: None

Package migration captures the facilities needed to define and execute
migrations for a crdb cluster. These migrations can be arbitrarily long
running, are free to send out arbitrary requests cluster wide, change
internal DB state, and much more. They're typically reserved for crdb
internal operations and state. Each migration is idempotent in nature,
is associated with a specific cluster version, and executed when the
cluster version is made activate on every node in the cluster.

Examples of migrations that apply would be migrations to move all raft
state from one storage engine to another, or purging all usage of the
replicated truncated state in KV. A "sister" package of interest is
pkg/sqlmigrations.

---

This commit only introduces the basic scaffolding and wiring from
existing functionality. We'll flesh in the missing bits in future
commits.

Release note: None

The migration manager will make use of the EveryNode RPC once it's
properly wired up (in future commits). It'll need a node dialer to do
so.

Release note: None

We expect to add multiple req/resp types as individual operations for
the EveryNode RPC. Each of these operations will correspond to a
"primitive" of sorts for the (long running) migrations infrastructure.
It's a bit cumbersome to wield this nested union type, so we
autogenerate helper code to do it for us. We take precedence from
api.proto and all the very many batch request/responses.

Release note: None

To flesh out the migrations infrastructure (in future commits), we'll
need a handle on all the nodes present in the system. Now that we always
create a liveness record on start up (cockroachdb#53805), we can simply fetch all
liveness records from KV. We add a helper to do so.

It's a bit unfortunate that we're further adding on to the NodeLiveness
API without changing the caching structure, but the methods fetching
records from KV is the world we're hoping to move towards going forward.

This does mean that we're introducing a direct dependency on
NodeLiveness in the sql layer, and there's improvements to be made here
around interfaces delineating between "system tenant only" sql code and
everything else. Only system tenants have the privilege to set cluster
settings (or at least the version setting specifically), which is what
this API will look to power.

Release note: None

Plumb in the view into node liveness that was fleshed out earlier. We
use it to power the RequiredNodes primitive, that provides a handle on
all nodes in the system.

Copying from elsewhere:

    // RequiredNodes returns the node IDs for all nodes that are
    // currently part of the cluster (i.e. they haven't been
    // decommissioned away). Migrations have the pre-requisite that all
    // required nodes are up and running so that we're able to execute
    // all relevant node-level operations on them. If any of the nodes
    // are found to be unavailable, an error is returned.

Release note: None

This commit is not going to be merged. It was added to test things
quickly (in lieu of actual tests) by letting me bump versions willy
nilly (ignoring max allowable version).

Release note: None

We'll use this primitive in a future commit to introduce additional
safeguards (not present today) around cluster version upgrades.
Specifically, we'll use this EveryNode operation to validate that every
node in the cluster is running a binary that's able to support the
specified cluster version.

...in favor of direct RPCs to all nodes in the cluster. This commit in
particular deserves a bit of scrutiny. It uses the building blocks we've
added thus far to replace the use of gossip to disseminate the cluster
version. It does so by sending out individual RPCs to each node in the
cluster, informing them of a version bump, all the while retaining the
same guarantees provided by our (now previously) gossip-backed mechanism.

This diff has the following "pieces":
- It disconnecting version setting updates through gossip (by
  disconnecting the setting type within the updater process)
- It using the EveryNode RPC to send out RPCs to each node in the
  cluster, containing the payload that each node would otherwise receive
  through gossip.
- It expands the clusterversion.Handle interface to allow setting the
  active version directly through it.
- It persisting any cluster versions received from other nodes first,
  within keys.StoreClusterVersionKey, before bumping the version gate.
  This was previously achieved by attaching callbacks on the version
  handle (look towards all the diffs around SetBeforeChange). This is an
  invariant we also maintained earlier.
- It using the active version provided by the join RPC to set the
  version setting directly (after having persisted it first). This too
  was previously achieved through gossip + the callback.

While here, we add a few comments and chip away at the callback hooks
that are no longer needed.

Release note: None

Just filling in a few missing dependencies we expect migration code to
rely on going forward.

Release note: None

It's not currently wired up to anything (there are no real migrations
yet), but it's one of the primitives we expect future migrations to rely
on (in future commits).

Release note: None

We'll use it in a future commit to store migration state, for
introspection.

> SHOW CREATE system.migrations;
         table_name        |                     create_statement
---------------------------+------------------------------------------------------------
  system.public.migrations | CREATE TABLE public.migrations (
                           |     id INT8 NOT NULL DEFAULT unique_rowid(),
                           |     metadata STRING NOT NULL,
                           |     started TIMESTAMP NOT NULL DEFAULT now():::TIMESTAMP,
                           |     progress BYTES NULL,
                           |     CONSTRAINT "primary" PRIMARY KEY (id ASC),
                           |     FAMILY "primary" (id, metadata, started),
                           |     FAMILY progress (progress)
                           | )

Release note: None

This reverts an earlier commit removing version upgrade safeguards. This
commit will also not be merged.

Release note: None