MB-11462 Acknowledge bytes received for close requested streams

When a close_stream request is send to the upr server, by that
time upr server would have already placed some mutations in
its network buffer for that stream. The upr client has to read
those mutations from the socket and ignore them. But even though
the client's user is not interested in it, since we received those
bytes we have to acknowledge using flow control ack message.

Whenever a high priority task arrives, the view engine cancels
an updater job and sends close stream message for currently active
upr stream. Since we did not send ack message for closed stream's
received bytes, connection was getting stuck when upr flow control
window gets full.

Change-Id: Ieedc664c9716e5e38c00852c1c35056649e69e6c
Reviewed-on: http://review.couchbase.org/38636
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>