implemented ns_server side of _metakv endpoint

Change-Id: Ic67389cd1371a6c04d645178701e17dc1d09e2a0
Reviewed-on: http://review.couchbase.org/44454
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>