Add a preliminary consistent store

I'm from the "required" bad school ('cos we might want to change the schema and there will definitely be serialized versions of these kicking around...)

You're right that the node id will probably be the key for these, but I put it in with vague thoughts of a failsafe against accidentally updating someone else's node state.  (but hey, it's optional so we can remove it later on if it's not actually all that helpful :D)