Request an ack iff backfill queue is empty during backfill phase.

Change-Id: I2ca5d708d3d0375d6e8e39e4f92c9ecb8204e24b
Reviewed-on: http://review.couchbase.org/13390
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>

tapconnection.cc

@@ -530,7 +530,7 @@ bool TapProducer::requestAck(tap_event_t event, uint16_t vbucket) {
 
     return explicitEvent ||
            (seqno - 1) % ackInterval == 0 || // ack at a regular interval
-           (!backfillCompleted && getBackfillQueueSize_UNLOCKED() < 100) ||
+           (!backfillCompleted && getBackfillQueueSize_UNLOCKED() == 0) ||
            empty_UNLOCKED(); // but if we're almost up to date, ack more often
 }