title: Celery workers not filling concurrency slots
body: <p>I have one worker with a concurrency of 4.  I see 4 processes started in flower and everything looks good.</p>
  
  <p>If I do this in a shell then I see the 4 workers take the tasks and the rest are reserved and it processes 4 at a time until the queue is empty.</p>
  
  <pre><code>[my_task.apply_async() for i in xrange(10)]
  </code></pre>
  
  <p>However if I do this line by line only the first two tasks gets actively worked on and from then on out it processes only two at a time.</p>
  
  <pre><code>my_task.apply_async()
  my_task.apply_async()
  my_task.apply_async()
  my_task.apply_async()
  ...
  </code></pre>
  
  <p>Any ideas?</p>