Stricter typing/serialization for state

[duijf]

Up until now we've gotten away with using the state.json file in the root of buckets. This is an unsustainable design: we won't get away with this for much longer.

I'd like to switch to SQLite for this stuff. We want stricter schemas, validation, and migrations if we're actually going to put pxl to use in a production setting and handle upgrades across versions. I also have some nice ideas about resumable uploads