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>

notifications/listener_test.go

@@ -18,7 +18,7 @@ import (
 
 func TestListener(t *testing.T) {
 	ctx := context.Background()
-	registry := storage.NewRegistryWithDriver(ctx, inmemory.New(), memory.NewInMemoryBlobDescriptorCacheProvider())
+	registry := storage.NewRegistryWithDriver(ctx, inmemory.New(), memory.NewInMemoryBlobDescriptorCacheProvider(), true)
 	tl := &testListener{
 		ops: make(map[string]int),
 	}