MB-11924: added memcached_config_mgr

New process is responsible for writing memcached.json and actually
enabling startup of memcached. It also implements support for
reloading memcached config.

Change-Id: Ie4f8be8ecf39aec91cedcaba00979b41b79a4dbc
Reviewed-on: http://review.couchbase.org/42015
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>