title: Configure RabbitMQ to replace an old pending message with a new one
body: <p>Is is possible to configure a RabbitMQ exchange or a queue in such a way that at most one message with a given routing key is pending at any time? If new message arrives, the old one would be dropped and the new one enqueued. </p>
  
  <p>If such option is not available, what would be the best way to implement this at the application level? I.e. when application receives a message how can it check if there any more pending messages?</p>