BUG/MEDIUM: h2: Don't attempt to recv from h2_process_demux if we subscribed.

BUG/MEDIUM: h2: Don't attempt to recv from h2_process_demux if we subscribed.

Modify h2c_restart_reading() to add a new parameter, to let it know if it
should consider if the buffer isn't empty when retrying to read or not, and
call h2c_restart_reading() using 0 as a parameter from h2_process_demux().
If we're leaving h2_process_demux() with a non-empty buffer, it means the
frame is incomplete, and we're waiting for more data, and if we already
subscribed, we'll be waken when more data are available.
Failing to do so means we'll be waken up in a loop until more data are
available.

This should be backported to 1.9.