BUG/MINOR: mux-h2: advertise a larger connection window size

BUG/MINOR: mux-h2: advertise a larger connection window size

By default H2 uses a 65535 bytes window for the connection, and changing
it requires sending a WINDOW_UPDATE message. We only used to update the
window when receiving data, thus never increasing it further.

As reported by user klzgrad on the mailing list, this seriously limits
the upload bitrate, and will have an even higher impact on the backend
H2 connections to origin servers.

There is no technical reason for keeping this window so low, so let's
increase it to the maximum possible value (2G-1). We do this by
pretending we've already received that many data minus the maximum
data the client might already send (65535), so that an early
WINDOW_UPDATE message is sent right after the SETTINGS frame.

This should be backported to 1.8. This patch depends on previous
patch "BUG/MINOR: mux-h2: refrain from muxing during the preface".