This makes use of the fact that Varnish will always send a session window
update immediately when receiving a data frame. The four rxwinup before
barrier sync on stream 0 matches up with the four txdata frames sent early
on the stream, and makes sure that the session thread has exhausted its
send window completely before opening up and starting to consume rxbuf
data by unblocking the client thread stuck in vcl_recv. From that point on
window updates will also be sent on the stream.