Using Cache::primary_store to load a non-empty store

[eschkufz]

I'm fine with leaving it as undefined behavior if you modify the primary store once you've passed ownership of it to a cache. But when you do this, the cache should at the very least reset its policies (probably clear everything and then touch everything in the new primary store).

[eschkufz]

(Note that you don't have this same problem with AdapterStore since it doesn't maintain any state relative to the underlying store. This is a Cache-only problem, despite the similar implementation.)