BUG/MEDIUM: mux-h2: don't needlessly wake up the demux on short frames

BUG/MEDIUM: mux-h2: don't needlessly wake up the demux on short frames

In some situations, if too short a frame header is received, we may leave
h2_process_demux() waking up the task again without checking that we were
already subscribed.

In order to avoid this once for all, let's introduce an h2_restart_reading()
function which performs the control and calls the task up. This way we won't
needlessly wake the task up if it's already waiting for I/O.

Must be backported to 1.9.