imgstore: a simplified, redesigned interface for an image store

For early discussion. This is a mockup of a new interface to manage container images. It would replace graph/ and image/.

See also: #6802 and #6805

Signed-off-by: Solomon Hykes <solomon@docker.com>
I like this, though does installing a new image mean the old image is popped off?

I would think this is where would have an incrementor to show that you've got prior images of the same <repo>:<tag> that could be rolled'back to.