title: Restrict number of instances of a build in the queue
body: <p>Is it possible to limit the number of instances of a single build configuration in the queue?</p>
  
  <p>When a build is triggered I would like it to be added to the queue but if there are more than x instances of that build on the queue already then the oldest instance of the build in the queue is removed as well as the new instance being added.</p>
  
  <p>Is there a plugin that does this?</p>