MEDIUM: filters/http: Move body parsing of HTTP messages in dedicated functions

MEDIUM: filters/http: Move body parsing of HTTP messages in dedicated functions

Now body parsing is done in http_msg_forward_body and
http_msg_forward_chunked_body functions, regardless of whether we parse a
request or a response.
Parsing result is still handled in http_request_forward_body and
http_response_forward_body functions.

This patch will ease futur optimizations, mainly on filters.