Fix object reloading bug

@byroot @camilo

I found two issues causing code reloading to fail in dev mode that were introduced in 0.1.1 in: https://github.com/byroot/frozen_record/commit/b5727d16b855231c7503d76de345be6187597b89

The first was that the instance of `store` being a `ThreadSafeStorage` was not correctly memoized. The side effect of this is primarily that we are building a new instance of this object in production about a dozen times each request in production right now, if not more.

The other is that instances of the class are being stored on `Thread.current`. Which is fine. But in dev mode in Rails when a ruby file is modified rails calls undefs and loads the class and builds a new one. But since we would on next request retrieve the memozied instance of the class from `Thread.current` we would end up with an instance of a class that no longer existed.

The specific manifestation we were seeing of this issue was that when one of our models that inherrits from `FrozenRecord::Base` called `super` inside a method it was looking for a class that no-longer exists.

The fix is straightforward. It keys by the `ThreadSafeStorage#object_id` and the name of the class. This leaks a bit, but only in dev mode. Anything we do in prod will dump the current thread context.

I struggled to write a test for this but ultimately decided it was too internal. All other tests continue to work.
Happens. It wasn't destructive or damaging. Mostly it's wasteful, which is funny since the entire point of this is to speed up loading. Heh.