MEDIUM: h1-htx: Split function to parse a chunk and the loop on the buffer

MEDIUM: h1-htx: Split function to parse a chunk and the loop on the buffer

A generic function is now used to only parse the current chunk (h1_parse_chunk)
and the main one (h1_parse_msg_chunks) is used to loop on the buffer and relies
on the first one. This change is mandatory to be able to use an optimized
function to parse contiguous small chunks.