Add XDCR_USE_NEW_PATH environment variable for using UPR codepath

Change-Id: I9f870369256dce711463dcf8d9b47101be4a7de0
Reviewed-on: http://review.couchbase.org/35107
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Reviewed-by: Tommie McAfee <tommie@couchbase.com>
Reviewed-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>
Reviewed-by: Aruna Piravi <aruna@couchbase.com>
Tested-by: Aruna Piravi <aruna@couchbase.com>