Factor loader into a database storer and a batch loader

ManifestURL is defensively exposed in case somewhere else wants it; mapping from loadUuid and bucket to manifest name isn't available otherwise

LoadChecker{} being in its original package makes it a circular dependency with metadata. I suppose it's ok if I define it in the same package as it's used in.

Logger can be simplified per your earlier suggestion. Stats is a huge pain, and I don't want to just dupe the code.

Let's talk about the overall package abstraction tomorrow morning.