Dismantle `lib/models`.

 - Move core models `Model` and `PersistedModel` to `lib/`.
 - 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).
Auto-load and register built-in `Checkpoint` model
test: remove infinite timeout

The infinite timeout was useful when debugging, which is not a good
reason for keeping it around when not debugging.
test: verify exported models
models: move User LDL def into `user.json`