replicator: Fix user placement in replication queue

replicator: Fix user placement in replication queue

Especially replicator queue importing and NOTIFY command could have
placed the user to wrong place in the queue, because they modified the
last sync timestamps afterwards.

Fixed by splitting replicator_queue_add() into get/update/add(), so all
the necessary changes can be done before replicator_queue_add() is used
to place the user into the queue.