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>

blobs.go

@@ -97,6 +97,11 @@ type BlobDeleter interface {
 	Delete(ctx context.Context, dgst digest.Digest) error
 }
 
+// BlobEnumerator enables iterating over blobs from storage
+type BlobEnumerator interface {
+	Enumerate(ctx context.Context, ingester func(dgst digest.Digest) error) error
+}
+
 // BlobDescriptorService manages metadata about a blob by digest. Most
 // implementations will not expose such an interface explicitly. Such mappings
 // should be maintained by interacting with the BlobIngester. Hence, this is