MB-23505: Add test for notification upon receiving a backfill item
MB-23505: Add test for notification upon receiving a backfill item

Test case for commit 78a65deb6a14150829c44b06fcd8d53d5389f1ba.

It tests if replica vbucket notifies pending high priority requests
upon receiving backfill items.

Change-Id: Ief9e14c265081fab70cc2823a410d6f0c84cd477
Reviewed-on: http://review.couchbase.org/76152
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

membase/ep-engine,membase/ep-engine,daverigby/kv_engine,daverigby/kv_engine,couchbase/ep-engine,couchbase/ep-engine,membase/ep-engine,membase/ep-engine,daverigby/kv_engine,couchbase/ep-engine,couchbase/ep-engine,daverigby/kv_engine