world: PEP 8.
world: Start factoring out caching behavior.

Now the permanent cache is a separate object which has its own rules.

Also added a handful of tests and moved permanent cache initialization into
World and out of the Factory. It's a detail of World that nobody else should
see.

Refs #366.
world: Make enable_chunks() return a Deferred, and test.

This test actually shows off some pretty deep codepaths.

Refs #366.