MB-5307 Use define for ns_server logger category.

Just to be consistent.

Change-Id: I876cd21935417e557ff6a60812ea04c2070a7106
Reviewed-on: http://review.couchbase.org/26045
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>