introduced xdcr_sup

Change-Id: I6cdc497375729a181e4b176bded547dd64ed9e3c
Reviewed-on: http://review.couchbase.org/40088
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>