Add locking for multithread applications

[cfxegbert]

Something like

if (proxy := getattr(self, "_ContainerProxy__proxy_object")) is None:
    with getattr(self, "_ContainerProxy__lock"):
        if (proxy := getattr(self, "_ContainerProxy__proxy_object")) is None:
            proxy = getattr(self, "_ContainerProxy__supplier")()
            super().__setattr__("_ContainerProxy__proxy_object", proxy)

Looks like __create_instance will also need updated with locking.