feat: if the event queue a shutdown event, always pick it first — radicle-ci-broker

Rationale: we don’t want to wait for a long time until shutdown starts.

         let mut queue = vec![];
         for id in ids.iter() {
             if let Some(qe) = self.db.get_queued_ci_event(id).map_err(QueueError::db)? {
                 #[allow(clippy::single_match)]
                 match qe.event() {
                     CiEvent::V1(CiEventV1::Shutdown) => return Ok(Some(qe.clone())),
                     _ => (),
                 }
                 queue.push(qe);
             }
         }