MAJOR: mux-h1: Remove the rxbuf and decode HTTP messages in channel's buffer

MAJOR: mux-h1: Remove the rxbuf and decode HTTP messages in channel's buffer

It avoids a copy between the rxbuf and the channel's buffer. It means the
parsing is done in h1_rcv_buf(). So we need to have a stream to start the
parsing. This change should improve the overall performances. It also implies a
better split between the connection layer and the applicative layer. Now, on the
connection layer, only raw data are manipulated. Raw data received from the
socket are stored in ibuf and those sent are get from obuf. On the applicative
layer, data in ibuf are parsed and copied into the channel's buffer. And on the
other side, those structured data are formatted and copied into obuf.