Implements garbage collection subcommand

- Includes a change in the command to run the registry. The registry
  server itself is now started up as a subcommand.
- Includes changes to the high level interfaces to support enumeration
  of various registry objects.

Signed-off-by: Andrew T Nguyen <andrew.nguyen@docker.com>

docs/proxy/proxyregistry.go

@@ -166,6 +166,14 @@ func (pr *proxyingRegistry) Repository(ctx context.Context, name reference.Named
 	}, nil
 }
 
+func (pr *proxyingRegistry) Blobs() distribution.BlobEnumerator {
+	return pr.embedded.Blobs()
+}
+
+func (pr *proxyingRegistry) BlobStatter() distribution.BlobStatter {
+	return pr.embedded.BlobStatter()
+}
+
 // authChallenger encapsulates a request to the upstream to establish credential challenges
 type authChallenger interface {
 	tryEstablishChallenges(context.Context) error