Use model reference from env when it is available

[EiNSTeiN-]

The get_session_model method will store a reference to the model in request.env[SESSION_RECORD_KEY] when the session is loaded. Later on when saving the session, write_session calls get_session_model, which again calls the active record finder methods to load the model.

In a standard Rails app, this would presumably hit the active record cache and not hit mysql twice, but if the active record cache is invalidated during the request or if @@session_class#find_by_session_id was overloaded to do anything else (like perhaps read from an external cache), then the app will end up doing unnecessary work.