allow enchancers for sockets in xmem connection pools

We need this for things like "this socket needs datatype extention to
be enabled"

Change-Id: I5347bca250306c36295c72512d57d0ba8045d731
Reviewed-on: http://review.couchbase.org/37560
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>