Improve and fix runner API

- The server now also signals abort_work if the commit is no longer in
  the queue.
- The server now immediately reserves the work it gives to the worker,
  so other idle workers won't get the same work.
- The server now chooses a run id, not the worker. The worker can still
  submit work under arbitrary run ids when no id is known, for example
  when importing runs from another database.

src/worker/server.rs

@@ -161,7 +161,7 @@ impl Server {
             assert!(!unfinished);
             assert!(self.run.is_none());
 
-            let run = Arc::new(Mutex::new(Run::new(work.hash)));
+            let run = Arc::new(Mutex::new(Run::new(work.id, work.hash)));
             let (abort_tx, abort_rx) = mpsc::unbounded_channel();
 
             self.run = Some((run.clone(), abort_tx));