(sync_part): Use ldifftime() instead of difftime().
(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.