Keep track of how many threads are waiting

src/hydra-queue-runner/state.hh

@@ -203,6 +203,7 @@ private:
     counter nrStepsBuilding{0};
     counter nrStepsCopyingTo{0};
     counter nrStepsCopyingFrom{0};
+    counter nrStepsWaiting{0};
     counter nrRetries{0};
     counter maxNrRetries{0};
     counter totalStepTime{0}; // total time for steps, including closure copying