MEDIUM: http: make the chunk crlf parser only depend on the buffer

MEDIUM: http: make the chunk crlf parser only depend on the buffer

The chunk crlf parser used to depend on the channel and on the HTTP
message, eventhough it's not really needed. Let's remove this dependency
so that it can be used within the H2 to H1 gateway.

As part of this small API change, it was renamed to h1_skip_chunk_crlf()
to mention that it doesn't depend on http_msg anymore.