title: Task throttling not working with ConcurrentExclusiveSchedulerPair
body: <p>I am trying to throttle the number of async tasks that I submit to an external database server.  I created the following test code which should take 25 seconds to execute (150 tasks * 5 seconds / 30 max concurrent tasks = 25 seconds).  </p>
  
  <pre><code>    private async void ThrottlingTest()
      {
          TaskScheduler scheduler = new ConcurrentExclusiveSchedulerPair(TaskScheduler.Default, 30).ConcurrentScheduler;
          Stopwatch sw = new Stopwatch();
  
          int numTasks = 150;
          Task[] tasks = new Task[numTasks];
  
          for (int i = 0; i &lt; numTasks; i++)
          {
              tasks[i] = Task.Factory.StartNew(() =&gt; Task.Delay(5000), CancellationToken.None, TaskCreationOptions.None, scheduler).Unwrap();
          }
  
          sw.Start();
          await Task.WhenAll(tasks);
          sw.Stop();
          long duration = sw.ElapsedMilliseconds;
      }
  </code></pre>
  
  <p>However, this code only takes 5 seconds to execute indicating my throttling is not working.  What am I doing wrong?  I suspect Task.Delay is running using the default scheduler.  If so, what delay mechanism can I test with?</p>