Use sha256 for image ids, eliminate id in JSON

This patch currently aims to provide compatibility for loading
older image formats but does NOT intend for older clients to support
loading the newer image format (as due to chicken-and-egg, we
can no longer put the image id into the JSON data)

Closes-Bug: #6959

Docker-DCO-1.1-Signed-off-by: Eric Windisch <ewindisch@docker.com> (github: ewindisch)