MB-11924: moved ns_memcached_sockets_pool earlier in start order

Change-Id: Ie9cc7c0d9dfef893fa5b9c4b3537a9664c55be67
Reviewed-on: http://review.couchbase.org/42013
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>