adding xdcr backfill prioritization test
adding xdcr backfill prioritization test

Change-Id: Ie71e0a22c84292f146f791fe0729ac487c05ce14
Reviewed-on: http://review.couchbase.org/107316
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Alex Gyryk <0e5cab161284683621f17481c0995df0d35b63eb@couchbase.com>

couchbase/perfrunner,couchbase/perfrunner,couchbase/perfrunner,couchbase/perfrunner,couchbase/perfrunner,couchbase/perfrunner