MEDIUM: h2: perform a single call to the data layer in demux()

MEDIUM: h2: perform a single call to the data layer in demux()

Instead of calling the data layer from each individual frame processing
function, we now call it from demux. This requires to know the h2s that
was created inside h2c_frt_handle_headers(), which is why the pointer is
now returned. This results in a small performance boost from 58k to 60k
POST requests/s compared to -master, thanks to half the number of
si_cs_recv_cb() calls and 66% calls to si_cs_wake_cb().

It's interesting to note that all calls to data_cb->recv() are now always
immediately followed by a call to data_cb->wake(). The next step should
be to let the ->wake handler perform the recv() call itself. For this it
will be useful to have some info on the CS to indicate whether or not it
is ready to be read (ie: contains a non-empty input buffer).