[MEDIUM] session: finish session establishment sequence in with I/O handlers

[MEDIUM] session: finish session establishment sequence in with I/O handlers

Calling sess_establish() upon a successful connect() was essential, but
it was not clearly stated whether it was necessary for an access to an
I/O handler or not. While it would be desired, having it automatically
add the response analyzers is quite a problem, and it breaks HTTP stats.

The solution is thus not to call it for now and to perform the few response
initializations as needed.

For the long term, we need to find a way to specify the analyzers to install
during a stream_int_register_handler() if any.