/**
* Returns the next operation that must be ran
* @return mm\Daemon\Operation
*/
public function next()
{
// maintain count of how many operations of each type are currently running
// depending on the priority (#1 = downloads, #2 = merge), return the next operation
// when an operation finishes, the slot is cleaned up, and one more of the same type can resume
return Queue::getNextItem();
}