fix performance issue, round robined (persistent) queues should not wait

[bendavies]

This fixes a performance issue where by persistent queues, with drivers that respect $duration on popMessage.

This issue is that, during a round robin, an empty persistent queue will wait for 5 seconds before allowing the next queue to dequeue. The problem is obviously compounded the more queues in the round robin.

[henrikbjorn]

This changes the logic when using the round robin, instead the sleep should be equal to the $duration

[bendavies]

hi @henrikbjorn

I'm not really sure what you mean. RoundRobin does not have any $duration.
The sleep is to prevent 100% CPU usage when iterating the queues.

The main issue is rather more involved here.

1. Add $duration parameter to PersistentQueue.dequeue() #326 should not have been merged,$duration should have been added at the interface level.
2. Not all the drivers implement $duration in popMessage.

These needs fixing really but would be a large job.

This PR is the quickest way to fix the round robin queue, which is currently pretty useless when used with multiple persistent queues.

[acrobat]

Related to #321