This is important for distributed settings, and will make merging
databases much simpler. This change is pretty extensive and includes a
lot of other stuff like moving some fields between tables, and
introducing the `local_metadata` table which is a key-value store and
holds the uuid of the "local" store, i.e. the one corresponding to the
directory holding that particular nancy.db.
The record() program and cli is not yet working. For that we need a
working `FSDiff` next. Then we'll finally be ready to start recording
file information.
