Refactor storage to use new backend layout

This change refactors the storage backend to use the new path layout. To
facilitate this, manifest storage has been separated into a revision store and
tag store, supported by a more general blob store. The blob store is a hybrid
object, effectively providing both small object access, keyed by content
address, as well as methods that can be used to manage and traverse links to
underlying blobs. This covers common operations used in the revision store and
tag store, such as linking and traversal. The blob store can also be updated to
better support layer reading but this refactoring has been left for another
day.

The revision store and tag store support the manifest store's compound view of
data. These underlying stores provide facilities for richer access models, such
as content-addressable access and a richer tagging model. The highlight of this
change is the ability to sign a manifest from different hosts and have the
registry merge and serve those signatures as part of the manifest package.

Various other items, such as the delegate layer handler, were updated to more
directly use the blob store or other mechanism to fit with the changes.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

storage/paths.go

@@ -188,7 +188,7 @@ func (pm *pathMapper) path(spec pathSpec) (string, error) {
 			return "", err
 		}
 
-		return path.Join(root, "current/link"), nil
+		return path.Join(root, "current", "link"), nil
 	case manifestTagIndexPathSpec:
 		root, err := pm.path(manifestTagPathSpec{
 			name: v.name,