replication: delay initial join until confirmation

All the data that master sends during the join stage (both initial and
final) is embedded into the first snapshot created on replica, so this
data mustn't contain any unconfirmed or rolled back synchronous
transactions.

Make sure that master starts sending the initial data, which contains a
snapshot-like dump of all the spaces only after the latest synchronous
tx it has is confirmed. In case of rollback, the replica may retry
joining.

Part of #5097