applier: treat register txns like regular ones

Applier during the registration waiting (for registering a new ID
or a name) could keep doing the master txns received before the
registration was started. They could still be inside WAL doing a
disk write, when the replica sends a register request.

Before this commit, it could cause an assertion failure in debug
and a double LSN error in release.

The reason was that during the registration waiting the applier
treated all incoming txns as "final join" txns. I.e. it wasn't
checking if those txns were already received, but not committed
yet.

During normal subscribe process the appliers (potentially
multiple) protect themselves from that by keeping track of the
vclocks which are already applied and also being applied right now
(replicaset.applier.vclock).

Such protection ensures that receiving same row from 2 appliers
wouldn't result into its double write. It also protects from the
case when a txn was received, goes to WAL, but then the applier
reconnects, resubscribes, and gets the same txn again - it
shouldn't be applied.

The patch makes so that the registration waiting after recovery
works like subscribe. Registration during recovery would mean
bootstrap via join. And outside of recovery it means the instance
is already running.

Closes #9916

NO_DOC=bugfix