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
Reviewed-on: http://review.couchbase.org/42009
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>