CBD-777: calculate XDCR stats for litmus dahboard

Change-Id: I5c4871a36d71fc364c7871556ea6de427d80258e
Reviewed-on: http://review.couchbase.org/24209
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Ronnie Sun <ronnie@couchbase.com>