title: python queuelib: Empty and clear out FifoDiskQueue
body: <p>I'm using <a href="https://pypi.python.org/pypi/queuelib" rel="nofollow noreferrer">queuelib</a> and so far it's great!</p>
  
  <p>Here is the repo:
  <a href="https://github.com/scrapy/queuelib" rel="nofollow noreferrer">https://github.com/scrapy/queuelib</a></p>
  
  <p>Is there a way to empty out the FifoDiskQueue?</p>