MB-8013 Notify rebalance start/end for memcached buckets too.

Change-Id: Iaad86e209a9b49ce663ffdc9b10f90d77cfddd95
Reviewed-on: http://review.couchbase.org/25907
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>