Description of change
The following commit is just a very rough idea around marking and sweeping values in the model cache in the case that the cache can have old stale items in it.
The problem this fixes is when the watcher restarts, but we have old stale content in the cache. So we should clean up anything that hasn't been updated.
Note this is very rough and I'm awaiting feedback before I can continue on wards.
QA steps
Please replace with how we can verify that the change works?
该提问来源于开源项目：juju/juju
