Stabilize r02679
Stabilize r02679

Tune down h2_rx_window_low_water to make sure we don't get a
window_update racing against the response frames.

Fixes: #2709

gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache