Allow a queue worker callback to be a class method

As we turn more and more to OOP this makes sense, doesnt it?:)

drupal_cron_run() right now:

      $function = $info['worker callback'];
      $end = time() + (isset($info['cron']['time']) ? $info['cron']['time'] : 15);
      $queue = Drupal::queue($queue_name);
      while (time() < $end && ($item = $queue->claimItem())) {
        $function($item->data);
        $queue->deleteItem($item);
      }

Coming from #1972260: Convert aggregator_admin_refresh_feed to a new style controller