MEDIUM: h2: start to consider the H2_CF_{MUX,DEM}_* flags for polling

MEDIUM: h2: start to consider the H2_CF_{MUX,DEM}_* flags for polling

Now we start to set the flags to indicate that the response buffer is
being awaited or that it is full, it makes it possible to centralize a
little bit the polling management into the wake() callback.

In case of error, we wake all the streams up so that they are aware of
the nature of the event and are able to detach if needed.