Remove memoization

[johnnyshields]

I think a lot of the memoization (at least in Trackable class) should be removed. Given that Mongoid already caches queries, I don't think it actually reduces db hits, and moreover it seems to be cleared frequently. I think it mainly adds complexity and potential for bugs. Was there a real use case for implementing it?

[dblock]

I doubt that this was ever intentional.

[aq1018]

I actually did do this intentionally because I didn't know for sure if the queries are cached. I guess I pre-optimized a bit too much. Feel free to remove as you see fit.