[CLEANUP] move the session-related functions to session.c

[CLEANUP] move the session-related functions to session.c

proto_http.c was not suitable for session-related processing, it was
just convenient for the tranformation.

Some more splitting must occur: process_request/response in proto_http.c
must be split again per protocol, and the caller must run a list.

Some functions should be directly attached to the session or the buffer
(eg: perform_http_redirect, return_srv_error, http_sess_log).