Split Cache module into ArtifactCache, BankCache, and possibly InventoryCache

[rdw-software]

The Cache module after its recent rework is mostly the (formerly local) artifact cache, but some code still remains that deals with the persistent bank cache. This should be separate and tested individually to make sure nothing goes wrong that wouldn't be covered by the artifact cache unit tests.

Roadmap:

• Split Cache into ArtifactCache and BankCache
• Write unit tests for the new BankCache API
• Optional: Create InventoryCache and do the same for that?
• Rethink and maybe update the remaining (original) Cache API? Some of the code is ridiculously awful as it hasn't really changed since it was first conceived.