backend: new `server_jid` option: Server domain must be known to validate requests, this can be done explicitely by using the `server_jid` option. If this option is not set, the server domain is found: - by using the `from` name of the initial delegation advertising message - or it fallbacks to using the part after initial `.` (`pubsub.example.org` would give `example.org`)

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