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
Ok, you have my permission to break everything. It's not like 3.1 -> 3.2 is going to be a smooth transition in any case.