Reduce CPU load depending on the queues priority, whereas priority
can be 1, 2 or 3 actually, so possible values for usleep are:
PriorityHigh: 100 === 0.0001 s
PriorityMedium: 10.000 === 0.01 s
PriorityLow: 1.000.000 === 1 s
See also:
AppserverIo\Appserver\Core\AbstractDaemonThread::getDefaultTimeout()