message: Don't spawn a new thread for fresh jobs
  The overhead in doing so is often much higher than the actual time it
  takes to execute the job
diff: @@ -880,10 +880,17 @@ impl<'cfg> DrainState<'cfg> {
  };
  match fresh {
  - Freshness::Fresh => self.timings.add_fresh(),
  - Freshness::Dirty => self.timings.add_dirty(),
  - }
  + Freshness::Fresh => {
  + self.timings.add_fresh();
  + // Running a fresh job on the same thread is often much faster than spawning a new
  + // thread to run the job.
  + doit();
  + }
  + Freshness::Dirty => {
  + self.timings.add_dirty();
  scope.spawn(move |_| doit());
  + }
  + }
  Ok(())
  }
  