MAJOR: mux-h1: Create the client stream as later as possible

MAJOR: mux-h1: Create the client stream as later as possible

This is the reason for all previous patches. The conn-stream and the
associated stream are created as later as possible. It only concerns the
frontend connections. But it means the request headers, and possibly the
first data block, are received and parsed before the conn-stream
creation. To do so, an embryonic H1 stream, with no conn-stream, is
created. The result of this "early parsing" is stored in its rx buffer, used
to fill the request channel when the stream is created. During this step,
some HTTP errors may be returned by the mux. It must also handle
http-request/keep-alive timeouts. A significative change is about H1 to H2
upgrade. It happens very early now, and no H1 stream are created (and thus
of course no conn-stream).

The most important part of this patch is located to the h1_process()
function. Because it must trigger the parsing when there is no H1
stream. h1_recv() function has also been simplified.