Extract memcached/membase bucket rebalance functions.
Extract memcached/membase bucket rebalance functions.

Change-Id: Ib0c240134570f9e1d92587f82812dee4b623290a
Reviewed-on: http://review.couchbase.org/59714
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Reviewed-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>

membase/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server