title: worker: Warm shutdown (MainProcess) after task started
body: <p>Setup: Celery 3.1, broker=RabbitMQ, backend=Redis.</p>
  
  <p>Scenario:
  Having a task is <code>state=STARTED (running)</code> my worker is being restarted. I'm getting <code>worker: Warm shutdown (MainProcess)</code> message (stdout). The worker successfully restarted but the task is stuck on <code>STARTED</code> state (monitored via flower) and nothing happen. </p>
  
  <p>Desired state: I wish that the stuck task will run again (or fail before the shutdown) - and not be ignored and left as 'STARTED' forever.</p>