vshard: rebalancer flag and mode

[TarantoolBot]

Related dev. issue(s): tarantool/vshard#432
Product: Tarantool/vshard
Since: 3.1, 0.1.25
Root document: https://www.tarantool.io/en/doc/latest/book/admin/vshard_admin/#configuration
SME: @ Gerold103

Details

So far it was impossible to specify which instance should run the
rebalancer. It was always automatically assigned using some
internal rules based on UUIDs.

Now the users can choose:

• Which specific instance should run the rebalancer. Can be a
  replica or a master - won't matter.
• In which replicaset the instance to run the rebalancer should be
  selected automatically.
• Which instances and whole replicasets should not run the
  rebalancer even when it is selected automatically.

For that there are 2 new options: rebalancer = <bool> and
rebalancer_mode = <name>.

The rebalancer flag can be either omitted, or set to true, or
false. It can be set for replicasets and for specific instances.
There can be only one rebalancer = true in the whole config. But
can be many rebalancer = false.

• rebalancer = true assigned to an instance means that this
  instance is guaranteed to run the rebalancer service on it.
  The instance role doesn't matter - it can be a replica or a
  master. Will run the rebalancer anyway.

• rebalancer = true assigned to a replicaset means that the
  service will run only on the master of this replicaset. Can be
  combined with master = 'auto' on the given replicaset.

• rebalancer = false assigned to an instance means that it will
  not run the rebalancer.

• rebalancer = false assigned to a replicaset means that all the
  instances of this replicaset will not run the rebalancer.

• rebalancer = nil (same as omitted, default) means that the
  instance/replicaset will be eligible to run the rebalancer
  only if rebalancer_mode = 'auto' is set and there are no
  rebalancer = true anywhere.

The option rebalancer_mode should be specified in the root of
the config. It can have one of those values:

• 'auto' - default. Means that the rebalancer service location
  is chosen automatically among all master instances in the
  cluster. Excluding those which have rebalancer = false on
  them or on their replicaset. If there are any
  rebalancer = true, then this mode works the same as
  'manual'.

• 'manual'. The rebalancer will run only if there is at least
  one rebalancer = true in the config. And only on the given
  instance / replicaset (depending on at which level the flag
  was specified - for a specific instance or for a whole
  replicaset).

• 'off'. The rebalancer will not run anywhere, regardless of all
  the rebalancer = true/false specified in the config.
  Requested by @ Gerold103 in tarantool/vshard@9fda354.