MB-12055: don't lose replica in other case of repl.-master switch

Previous fix in this area always expected past master to be dead. But
this was incorrect because rebalance may be stopped after updating
vbucket states to their post-move values, but prior to updating
vbucket map.

New fix takes this possiblity into account.

Change-Id: I31c2874f44d8257922f6474a5f021155ae8f92e7
Reviewed-on: http://review.couchbase.org/40886
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>