Empty the queue before to add new items

I'm sorry I sent you in the wrong direction.

Rather than not adding items to the queue, I think we should instead empty the existing items in the queue (only our items based on the queue name) when calling the drush command.
So if we have to do a second deployment shortly after the first, we will pre-heat the URLs of the second deployment.
Otherwise we will continue to pre-heat those of the first deployment, and all the URLs that had been pre-heated between the first and the second deployment would no longer be pre-heated.

I updated the issue's title to reflect that.

Automatically closed - issue fixed for 2 weeks with no activity.