CBD-220: establish ns_memcached connection asyncrhonously

So that shutdown during that phase is possible

Change-Id: I98d9c78328e3c1c8b297dca3a05c1f2f9aed166c
Reviewed-on: http://review.couchbase.org/25325
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>