Ack for unacked bytes when upr client has no active stream

When upr client completes processing of last stream_end message
for the last active stream, send flow control ack for the remaining
unacked bytes. This will help tests to identify whether client has
processed all bytes sent.

Change-Id: Ic797dc0e19c5ec8e46d4c1d769e2d81359f13aff
Reviewed-on: http://review.couchbase.org/38643
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>