relay: send heartbeat on subscribe if replica is uptodate

Currently, a realy sends a heartbeat message to the replica only if
there was no WAL events for 'replication_timeout' seconds. As a result,
a replica that happens to be uptodate on subscribe will not update the
lag until the timeout passes, which may delay configuration. Let's make
relay send a heartbeat message right after subscribe in case the replica
is uptodate.