MEDIUM: mux-h1: Add dummy mux to handle HTTP/1.1 connections

MEDIUM: mux-h1: Add dummy mux to handle HTTP/1.1 connections

For now, it is just an other kind of passthrough multiplexer, but with internal
buffers to be prepared to parse incoming messages and to format outgoing
ones. There is also a task attached to it to handle timeouts. However, because
it does not handle any timeout for now, this task is unused. And finally,
because it handles internal buffers, it also handles retries on recv/send. To
use this multiplexer, you must use the option "http-use-htx" both on the
frontend and the backend.

It does not support keep-alive and will freeze connections after the first
request/response.