psql: use `created` colum to sort by creation: `item_id` was used to sort by creation because it correspond to it and it's slighly more efficient, but #398 raises the use case of manually modified database to import items. The column has been changed to suit this use case, and `item_id` is now always added as last order, in case of date conflicts. This patch extends a contribution from Stephen Paul Weber (Singpolyma) from the ticket mentioned below. fix: 398

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