Serializes upload state to an HMAC token for subsequent requests

To support clustered registry, upload UUIDs must be recognizable by
registries that did not issue the UUID. By creating an HMAC verifiable
upload state token, registries can validate upload requests that other
instances authorized. The tokenProvider interface could also use a redis
store or other system for token handling in the future.

storage/manifeststore_test.go

@@ -153,6 +153,6 @@ func (mockedExistenceLayerService) Upload(name string) (LayerUpload, error) {
 	panic("not implemented")
 }
 
-func (mockedExistenceLayerService) Resume(uuid string) (LayerUpload, error) {
+func (mockedExistenceLayerService) Resume(lus LayerUploadState) (LayerUpload, error) {
 	panic("not implemented")
 }