WALObservers should be notified before the lock

TestReplication failed on the last 0.90 build because the log of the machine that was killed was rolled and got a few edits in before it was put in a ZK queue.

This fix is pretty easy, the notification should be sent before the lock is acquired instead of after that. It shouldn't have been like that since the beginning but I guess I missed it when I narrowed the lock.