MB-9644: do not allow updating bucket during rebalance

Otherwise our attempt to request janitor run for possible update of
replicas count crashes rebalance.

Change-Id: Iadf03e117cb35e7ab311d353a1a252e771873857
Reviewed-on: http://review.couchbase.org/30878
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>