MINOR: filters/http: Slightly update the parsing of chunks

MINOR: filters/http: Slightly update the parsing of chunks

Now, http_parse_chunk_size and http_skip_chunk_crlf return the number of bytes
parsed on success. http_skip_chunk_crlf does not use msg->sol anymore.

On the other hand, http_forward_trailers is unchanged. It returns >0 if the end
of trailers is reached and 0 if not. In all cases (except if an error is
encountered), msg->sol contains the length of the last parsed part of the
trailer headers.

Internal doc and comments about msg->sol has been updated accordingly.