This refactors the hmac state token to take control of the layerUploadState
json message, which has been removed from the storage backend. It also moves
away from the concept of a LayerUploadStateStore callback object, which was
short-lived. This allows for upload offset to be managed by the web application
logic in the face of an inconsistent backend. By controlling the upload offset
externally, we reduce the possibility of misreporting upload state to a client.
We may still want to modify the way this works after getting production
experience.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
