replication: set replica ID before _cluster commit

Replica registration works via looking for the smallest not
occupied ID in _cluster and inserting it into the space.

It works not so good when mvcc is enabled. In particular, if more
than 1 replica try to register at the same time, they might get
the same replica_id because don't see changes of each other until
the registration in _cluster is complete.

This in the end leads to all replicas failing the registration
except one with the 'duplicate key' error (primary index in
_cluster is replica ID).

The patch makes the replicas occupy their ID before they commit it
into _cluster. And new replica ID search now uses the replica ID
map instead of _cluster iterator.

This way the registration works like before - like MVCC does not
exist which is fine.

Closes #5601