Refactor storage API to be registry oriented

In support of making the storage API ready for supporting notifications and
mirroring, we've begun the process of paring down the storage model. The
process started by creating a central Registry interface. From there, the
common name argument on the LayerService and ManifestService was factored into
a Repository interface. The rest of the changes directly follow from this.

An interface wishlist was added, suggesting a direction to take the registry
package that should support the distribution project's future goals. As these
objects move out of the storage package and we implement a Registry backed by
the http client, these design choices will start getting validation.

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

storage/blobstore.go

@@ -18,8 +18,7 @@ import (
 // abstraction, providing utility methods that support creating and traversing
 // backend links.
 type blobStore struct {
-	driver storagedriver.StorageDriver
-	pm     *pathMapper
+	*registry
 }
 
 // exists reports whether or not the path exists. If the driver returns error