(pre_sync): Destruct cache_nodes for deleted conferences, persons and
	texts (thus making exists_b obsolete).
(throwable_p): Non-existing nodes are throwable even if they are dirty.
(limit_conf, limit_pers, limit_text_stat): Throw all throwable
	objects, not just the first throwable object.
(save_one_conf, save_one_pers, save_one_text): The fact that the
	node exists can be used instead of the exists_b flag.