Registers(or replaces) a handler invoked every time another client successfully adds or updates the specified key. The server pushes a notification for the added/updated key to all clients except the one that performed the change.Bydefault, the data store immediately calls back the client if the key value is known. This is recommended to prevent callingFetchAs()andRegisterChangeNotificationHandler()in a non-safe order where a key/value could be missed if it appears between a failed fetch and the successful registration of a handler.Bydefault, the handler will be called even if the key type expected by the client doesn't match the stored type. In such case, the handler optional value is not set.