txn_limbo: handle duplicate ACKs

Replica can send the same ACK multiple times. This is relatively
easy to achieve.

ACK is a message form the replica containing its vclock. It is
sent on each replica's vclock update. The update not necessarily
means that master's LSN was changed. Replica could write
something locally, with its own instance_id. Vclock is changed,
sent to the master, but from the limbo's point of view it looks
like duplicated ACK, because the received master's LSN didn't
change.

The patch makes limbo ignore duplicated ACKs.

Closes #5195
Part of #5219