replication: make replica subscribe to master's ballot

Previously replicas chose the remote master to boot from by comparing
master ballot, which are received in response to IPROTO_VOTE request
right on connection init.

Such information is not enough in some scenarios. For example, when
implementing anonymous replicas and retrying relica join, we had to
restart all connections in order to get the latest ballot information.

Let's change that: make replica subscribe to the built-in
"internal.ballot" event instead of relying on request-response scheme of
IPROTO_VOTE. Now replicas always have up-to-date ballot information and
there is no need to reinitialize connections to update the ballots.

Introduce a new fiber running in tx thread for this purpose: applier
ballot watcher. The fiber subscribes on "internal.ballot" event and
watches it all the time while the connection to master is alive.

In case the master isn't aware of IPROTO_WATCH request or of
"internal.ballot" event, old behaviour is also implemented: ballot
watcher simply waits for IPROTO_VOTE response and exits.

The ballot watcher is started whenever replica tries to connect or
reconnect to the remote master and is cancelled whenever its parent
connection to the master is closed.

We do not put much effort into restarting the fiber and retrying to
connect in case it fails. For now ballot info is only used during
bootstrap, and not trying to keep the fiber alive at all costs
simplifies the code quite a lot.

Later on ballot subscriptions will play a more significant role in
choosing the bootstrap leader: replicas will re-check remote ballots
every now and then during the bootstrap leader election.

Part-of #5272

NO_CHANGELOG=internal change
NO_TEST=tested by existing replication tests
NO_DOC=internal change