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

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

The chunk parser used to depend on the channel and on the HTTP message
but it's not really needed as they're only used to retrieve the buffer
as well as to return the number of bytes parsed and the chunk size.

Here instead we pass the (few) relevant information in arguments so that
the function may be reused without a channel nor an HTTP message (ie
from the H2 to H1 gateway).

As part of this API change, it was renamed to h1_parse_chunk_size() to
mention that it doesn't depend on http_msg anymore.