- Move core models `Model` and `PersistedModel` to `lib/`.
- Move `AccessContext` class to `lib/`, since it is not a model.
- Move all other built-in models to `common/models`.
This is a preparation for extracting model definitions to JSON files.
By splitting the change into multiple commits, git is able to keep track
of file moves (renames).
Fix the query in `Checkpoint.current()` to correctly specify sorting
`seq DESC`. Before this change, the first checkpoint was returned as the
current one.