MB-12037: don't loose replicas on stopped vbucket switch moves

The issue observed in MB-12037 is old bug in ns_janitor where it would
lose replicas after completed move if old master is among replicas of
new chain.

This fixes the problem and adds unit tests for this and related cases.

Change-Id: Ie6b19cd87230a3594d8953216b2749aed7e8ce9f
Reviewed-on: http://review.couchbase.org/40803
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>