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>
dropped support for writing port config files

Because it's not used anymore

Change-Id: I5869b84e894c303e74f3d7342707a2f7dfbd1d3f
Reviewed-on: http://review.couchbase.org/42016
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-11924: [api.txt] added description of memcached settings api

Change-Id: If9041e5d6534ef01d77a503760d81472e42e8579
Reviewed-on: http://review.couchbase.org/42017
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>