MEDIUM: mux-h1: Split h1_process_mux() to make code more readable

MEDIUM: mux-h1: Split h1_process_mux() to make code more readable

h1_process_mux() function was pretty huge a quite hard to debug. So, the
funcion is split in sub-functions. Each sub-function is responsible to a
part of the message (start-line, headers, payload, trailers...). We are
still relying on a HTTP parser to format the message to be sure to detect
errors.  Functionnaly speaking, there is no change. But the code is now more
readable.