Persistent storage of recorded data

[docwhat]

I've been trying to figure out how to make persistent storage "safe".

In the current model I track a timestamp for when an image was last seen by docker-gc. These timestamps are updated when:

• On startup, we scan all images (e.g. docker image --all)
• As events mention an image.
• Just prior to deleting old images, all container images are added.

The startup rule causes problems when persisting the state:

1. If we reset all timestamps on startup, then there is no advantage to persisting. All images have their timestamp reset.
2. If we don't, then events could have happened while docker-gc wasn't running and it could decide to remove things that are being used.

If we add cleaning of stopped containers (#3) then this gets even more tricky.

Another concern is removing items that aren't in docker anymore (e.g. images that have been docker rmi'd).