Verify restore to revision on MongoNS

Following the discussion on OAK-1339, the backup will be controlled from Mongo directly, but we still need to verify 2 things:

• ongoing transactions will be discarded on restore, the head has to point to the latest stable revision
• as an added benefit, the restore could happen to an older revision (see the sharded setup where a node can get ahead of the others between the moment the backup starts and when it will finish across the board)