Concurrent Queue

Adds a Drush command to run queues with concurrent processes.

Example usage:

# Run the 'example_queue' queue with five different processes, each running for up to 5 minutes.
drush queue-run-concurrent example_queue --concurrency=5 --override-time-limit=300

Drupal 7 queue items out of order

Note that Drupal 7 will not always guarantee that items added to the queue will be processed in a true first-in-first-out manner, especially if a large group of items were added to the queue at exactly the same time. If you find the queue operation items out of order, you may need to apply the core patch from #2285705: Drupal system queue doesn't work as a real queue to your local Drupal installation.