MEDIUM: h2: basic processing of HEADERS frame

MEDIUM: h2: basic processing of HEADERS frame

This takes care of creating a new h2s and a new conn_stream when a
HEADERS frame arrives. The recv() callback from the data layer is then
called to extract the frame into the stream's buffer. It is verified
that the stream ID is strictly greater than the known max stream ID.
And the last_id is updated if the current request is properly converted.
The streams are created in open or half-closed(remote) states.

For now there are some limitations :
  - frames without END_HEADERS are rejected (CONTINUATION not supported
    yet, will require some more changes so that the stream processor
    checks the H2 frame header by itself and steals the frames from the
    connection)
  - padding/stream_dep/priority are currently ignored
  - limited error handling, could be improved

But at least the request is properly decoded, transcoded and processed.