title: How to read messages in flight status and delete it by boto library
body: <p>For testing purpose, I need to read the messages in flight and delete the messages in flight. I can't find a suitable solution so I use work around way now </p>
  
  <p>to read messages in flight
   1. stop message consumer process
   2. wait for message is available and read it but not delete 
   3. start message consumer process</p>
  
  <p>to delete message in flight (try clear() but it only delete message available)
   1. delete queue
   2. recreate queue</p>
  
  <p>Any better solution?</p>