storage: changed items order: items were so far ordered by item_id, i.e. by order of creation. This has been changed to order them by date of last update, so an item overwritting an old one appears on top (with former behaviour, it was staying at the same place). The XEP is not clear on that, but this seems more in correct with § 6.5.7 which says "implementations SHOULD return the N most recent (as opposed to the N oldest) items".

create table callbacks (
    service text not null,
    node text not null,
    uri text not null,
    PRIMARY KEY (service, node, uri)
);