implemented disco items: - new getItemsIds method to get only ids of items, without payload or extra data - added delegation (PEP) data for disco items requests. It's a really dirty hack, but there is not clean way to do it beside rewriting the whole thing - authorized node items are returned on disco items when node is specified - if no node is specified, the nodes list is returned

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