MB-11924: added rest api for memcached settings

Change-Id: Ie9c87480682899c6ae0d824f37525814e6f3f518
Reviewed-on: http://review.couchbase.org/42010
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>