MB-10400: added xdcr_upr_sockets_pool

Change-Id: Ifcd228367684970b15150200cc83ff3451ff3079
Reviewed-on: http://review.couchbase.org/34813
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>