getQueuedBuilds(): Periodically stop to handle priority bumps

Previously, priority bumps could take a long time to get noticed if
getQueuedBuilds() was busy processing zillions of queue
additions. (This was made worse by the reintroduction of substitute
checking.)

src/hydra-queue-runner/state.hh

@@ -377,7 +377,7 @@ private:
     void queueMonitorLoop();
 
     /* Check the queue for new builds. */
-    void getQueuedBuilds(Connection & conn, std::shared_ptr<nix::StoreAPI> store, unsigned int & lastBuildId);
+    bool getQueuedBuilds(Connection & conn, std::shared_ptr<nix::StoreAPI> store, unsigned int & lastBuildId);
 
     /* Handle cancellation, deletion and priority bumps. */
     void processQueueChange(Connection & conn);