added ns_config_default:latest_config_marker()

To have dialyzer-verified way of getting latest-config-marker.

Change-Id: Ibed27dca669abf48e9418e79ee2fcb060ba0aa7a
MB-11924: moved memcached conns. limits to separate config fields

This commit also makes it global config key which appears to be more
practical, while still allowing per-node settings.

We also move memcached.json path to config.

Change-Id: I5133269dc6f433762002d3c87f662ab28e8ac139
MB-11924: added rest api for memcached settings

Change-Id: Ie9c87480682899c6ae0d824f37525814e6f3f518
MB-11924: added ns_memcached api for CMD_CONFIG_{VALIDATE,RELOAD}

Change-Id: Iaf2ed5c81dab5aec216e67e02707a8ee77599e06
MB-11924: added feature to prevent auto-starting of child ports

We're going to use it for memcached to give memcached.json config
manager process to do it's magic prior to actually starting memcached.

Change-Id: Ifdba3d4d430604ad1f7f82c7d07a5027e21dbb9c
MB-11924: moved ns_memcached_sockets_pool earlier in start order

Change-Id: Ie9cc7c0d9dfef893fa5b9c4b3537a9664c55be67
MB-11924: added ns_ports_setup:sync

So that we can wait until current round of ports synchronizations is
complete.

Change-Id: Id9a41e32d8ab7edb1678e084a4b4869718c55880
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
dropped support for writing port config files

Because it's not used anymore

Change-Id: I5869b84e894c303e74f3d7342707a2f7dfbd1d3f