Use the network read bytes for flow control

Currently message sizes are hardcoded.Now we have
changed that to use the network size length for the flow
control buffer ack.

Change-Id: I9c37a5068be943b31574464fb2edd3d2faf6a68d
Reviewed-on: http://review.couchbase.org/37954
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>