Ensure content store's can be entirely declarative

[joshrwolf]

Hauler can currently sync various types of declarative Content, but there isn't a guarantee that a store is entirely defined by a set of Contents. In other words, the store.Add() that sync uses is imperative, and is missing something like store.Remove().

[zackbradys]

Possibly still relevant? Part of the redesign of the store? @amartin120

[dweomer]

closing in favor of #202