MB-11130 Move blocking post move chores to single vbucket mover.

So that rebalance stop works if we get stuck in any of them.

Change-Id: I4f3b4795ea88f915f8a0c8c7cf8a2ea11670f902
Reviewed-on: http://review.couchbase.org/37135
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>