server: create new memdb table for storing system metadata

[rboyer]

This adds a new very tiny memdb table and corresponding raft operation
for updating a very small effective map[string]string collection of
"system metadata". This can persistently record a fact about the Consul
state machine itself.

The first use of this feature will come in a later PR.

[freddygv]

How come this accepts a slice rather than a single entry?

[rboyer]

I did not have a use case for multiple atomic updates at this time. It seemed somewhat cheap to make it possible to do multiple atomic updates in the future without having to resort to introducing either another raft operation integer code or to shim this into Txn.

Alternatively I could do it this way now:

type SystemMetadataRequest struct {
  Datacenter String
  Op SystemMetadataOp
  Entry *SystemMetadataEntry
  WriteRequest
}

And then should we need multi-write, we could just do it union style:

type SystemMetadataRequest struct {
  Datacenter String
  Op SystemMetadataOp

  // Only one of Entry or Entries may be specified.
  Entry *SystemMetadataEntry
  Entries []*SystemMetadataEntry

  WriteRequest
}

Thoughts?

[freddygv]

Yeah that is a possibility. I lean toward having a single entry per request and revisiting the txn path later.

[freddygv]

Left one last comment but will leave it up to you