MB-100: use faster module-less reference to ns_config inside it

Change-Id: Icc2846b0eba4d4c47078010b4524da19745f71b5
Reviewed-on: http://review.couchbase.org/27580
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>