wp: invalidate WpGlobal id when removed from registry
Once an object is removed from the registry, its id is invalidated and can be later reused by other objects. WpGlobal objects may be long-lived: e.g. those in tmp_globals may live over an add,remove,add sequence for the same id, and a new WpGlobal must be used for the second add. However, currently e.g. wp_registry_prepare_new_global may pick the WpGlobal for a previously removed object from the tmp_globals list. To address this, invalidate the WpGlobal id immediately when we get the registry remove event.