datastorecache: Add Locker field, public settings.

appengine/datastorecache/handler.go

Index: appengine/datastorecache/handler.go
diff --git a/appengine/datastorecache/handler.go b/appengine/datastorecache/handler.go
index 56c1ead7dbbe5e6f6e3d84c1858a4c7c4fd2cb36..00911bc0f01bf816febff8b24b7fb7d9e26d92d8 100644
--- a/appengine/datastorecache/handler.go
+++ b/appengine/datastorecache/handler.go
@@ -50,4 +50,11 @@ type Handler interface {
 	// caller. Otherwise, the return value will be used to update the cache
 	// entity.
 	Refresh(c context.Context, key []byte, v Value) (Value, error)
+
+	// Locker returns the Locker instance to use.
+	//
+	// The Locker is optional, and serves to prevent multiple independent cache
+	// calls for the same data from each independently refreshing that data. If
+	// Locker returns nil, no such locking will be performed.
+	Locker(c context.Context) Locker
 }