MEDIUM: mux-h2: rework h2_restart_reading() to differentiate recv and demux

MEDIUM: mux-h2: rework h2_restart_reading() to differentiate recv and demux

From the beginning, h2_restart_reading() has always been confusing because
it decides whether or not to wake the tasklet handler up or not. This
tasklet handler does two things, one is receiving from the socket to the
demux buf, and one is demuxing from the demux buf to the streams' rxbufs.

The conditions are governed by h2_recv_allowed(), which is also called at
a few places to decide whether or not to actually receive from the socket.
It starts to be visible that this leaves some difficulties regarding what
to do with possibly pending data.

In 2.0 with commit 3ca18bf0b ("BUG/MEDIUM: h2: Don't attempt to recv from
h2_process_demux if we subscribed."), we even had to address a special
case where it was possibly to endlessly wake up because the conditions
would rely on the demux buffer's contents, though the solution consisted
in passing a flag to decide whether or not to consider the buffer's
contents.

In 2.5 commit b5f7b5296 ("BUG/MEDIUM: mux-h2: Handle remaining read0 cases
on partial frames") introduced a new flag H2_CF_DEM_SHORT_READ which
indicates that the demux had to stop in the middle of a frame and cannot
make progress without more data. More adaptations later came in based on
this but this actually reflected exactly what was needed to solve this
painful situation: a state indicating whether to receive or parse.

Now's about time to definitely address this by reworking h2_restart_reading()
to check two completely independent things:
  - the ability to receive more data into the demux buffer, which is
    based on its allocation/fill state and the socket's errors
  - the ability to demux such data, which is based on the presence of
    enough data (i.e. no stuck short read), and ability to find an rx
    buf to continue the processing.

Now the conditions are much more understandable, and it's also visible
that the consider_buffer argument, whose value was not trivial for
callers, is not used anymore.

Tests stacking two layers of H2 show strictly no change to the wakeup
cause distributions nor counts.