Manifest and layer soft deletion.

Implement the delete API by implementing soft delete for layers
and blobs by removing link files and updating the blob descriptor
cache.  Deletion is configurable - if it is disabled API calls
will return an unsupported error.

We invalidate the blob descriptor cache by changing the linkedBlobStore's
blobStatter to a blobDescriptorService and naming it blobAccessController.

Delete() is added throughout the relevant API to support this functionality.

Signed-off-by: Richard Scothern <richard.scothern@gmail.com>

cmd/registry/config.yml

@@ -19,6 +19,8 @@ log:
         to:
           - errors@example.com
 storage:
+    delete:
+      enabled: true
     cache:
         blobdescriptor: redis
     filesystem: