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>

manifests.go

@@ -53,12 +53,18 @@ type ManifestService interface {
 	// Delete removes the manifest specified by the given digest. Deleting
 	// a manifest that doesn't exist will return ErrManifestNotFound
 	Delete(ctx context.Context, dgst digest.Digest) error
+}
 
-	// Enumerate fills 'manifests' with the manifests in this service up
-	// to the size of 'manifests' and returns 'n' for the number of entries
-	// which were filled.  'last' contains an offset in the manifest set
-	// and can be used to resume iteration.
-	//Enumerate(ctx context.Context, manifests []Manifest, last Manifest) (n int, err error)
+// ManifestEnumerator enables iterating over manifests
+type ManifestEnumerator interface {
+	// Enumerate calls ingester for each manifest.
+	Enumerate(ctx context.Context, ingester func(digest.Digest) error) error
+}
+
+// SignaturesGetter provides an interface for getting the signatures of a schema1 manifest. If the digest
+// refered to is not a schema1 manifest, an error should be returned.
+type SignaturesGetter interface {
+	GetSignatures(ctx context.Context, manifestDigest digest.Digest) ([]digest.Digest, error)
 }
 
 // Describable is an interface for descriptors