MB-11107: Ack at least every 5 secs to clear the upr flow ctl buffer

When there are no more ops being sent through replication we should
still ack whatever we have processed. If nothing has been acked
for 5 seconds we should send an ack.

Change-Id: I4248cd500d97e461797744fa682226ecde2c3a60
Reviewed-on: http://review.couchbase.org/37639
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>