import sat/wokkel modules only when making service: when importing some sat/wokkel module, a reactor is installed. This can cause trouble when using sat_pubsub along with other twisted plugin installing a specific reactor (which is the case for SàT backend and Libervia). This patch works around it by importing those modules only when making the service.

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