Implement immutable manifest reference support

This changeset implements immutable manifest references via the HTTP API. Most
of the changes follow from modifications to ManifestService. Once updates were
made across the repo to implement these changes, the http handlers were change
accordingly. The new methods on ManifestService will be broken out into a
tagging service in a later PR.

Unfortunately, due to complexities around managing the manifest tag index in an
eventually consistent manner, direct deletes of manifests have been disabled.

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

docs/handlers/app_test.go

@@ -84,7 +84,7 @@ func TestAppDispatcher(t *testing.T) {
 			endpoint: v2.RouteNameManifest,
 			vars: []string{
 				"name", "foo/bar",
-				"tag", "sometag",
+				"reference", "sometag",
 			},
 		},
 		{