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 imagined the checksumed ID would come from the Image as a func. Perhaps would could have an interface for what we expect, and then ensure that it marshals to JSON correctly to not corrupt.