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box: sync on replication configuration update

Now box.cfg() doesn't return until 'quorum' appliers are in sync not
only on initial configuration, but also on replication configuration
update. If it fails to synchronize within replication_sync_timeout,
box.cfg() returns without an error, but the instance enters 'orphan'
state, which is basically read-only mode. In the meantime, appliers
will keep trying to synchronize in the background, and the instance
will leave 'orphan' state as soon as enough appliers are in sync.

Note, this patch also changes logging a bit:
 - 'ready to accept request' is printed on startup before syncing
   with the replica set, because although the instance is read-only
   at that time, it can indeed accept all sorts of ro requests.
 - For 'connecting', 'connected', 'synchronizing' messages, we now
   use 'info' logging level, not 'verbose' as they used to be, because
   those messages are important as they give the admin idea what's
   going on with the instance, and they can't flood logs.
 - 'sync complete' message is also printed as 'info', not 'crit',
   because there's nothing critical about it (it's not an error).

Also note that we only enter 'orphan' state if failed to synchronize.
In particular, if the instnace manages to synchronize with all replicas
within a timeout, it will jump from 'loading' straight into 'running'
bypassing 'orphan' state. This is done for the sake of consistency
between initial configuration and reconfiguration.

Closes #3427

@TarantoolBot document
Title: Sync on replication configuration update
The behavior of box.cfg() on replication configuration update is
now consistent with initial configuration, that is box.cfg() will
not return until it synchronizes with as many masters as specified
by replication_connect_quorum configuration option or the timeout
specified by replication_connect_sync occurs. On timeout, it will
return without an error, but the instance will enter 'orphan' state.
It wil... (continued)

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