RequestFilter adjustments

Make the API more useful by allowing to override the response in pre-request filter so we can build rate-limiting, caching, 304 responses and so on.

Just return non-true from the pre-request handler, and the framework will try to parse it as a response value. Return SS_HTTPResponse to get that delivered to the user.

Needs some more testing - I've only checked against https://github.com/mateusz/controllerpolicy
Why not keep the parameters in the same order? (request, response, session, model).

Also, I see value in having well defined names (request over req, earlyResponse over earlyRes). What's the argument behind shortening these?