Extract JCR-agnostic resource interfaces out of FedoraResource/Datastream

Can we be sure that the type that is backing this resource is the same as the type that is backing its description? (I know that it's secretly going to be a JCR `Node` in both cases for the impl on the table, but I want to understand what kind of claim we're making about the relationship between the persistence for a thing and the persistence for its description.)