Better memoization/caching

[osolmaz]

Feedback from Eden:

The caching method is interesting, although I would've expected memoized responses first, then falling back to your file cache on miss.

I think you're also mixing caches between models, as they rely on the same file location. You likely want to either just have a session cache via memoised patterns - functools has a very simple one to add in - or you want to break up caches between models and services.