/**
  * Checks if workers are still valid.
  *
  * @param string $heartbeatAt
  * @return mixed|string
  */
 public function validateQueue($heartbeatAt)
 {
     if (microtime(true) > $heartbeatAt) {
         $this->queue->rewind();
         while ($this->queue->valid()) {
             $message = new MultipartMessage($this->workerChannel);
             $message->setBody("HEARTBEAT");
             $message->wrap($this->queue->key(), null);
             $message->send();
             $this->queue->next();
         }
         $heartbeatAt = microtime(true) + $this->config->getInterval();
     }
     $this->queue->purgeQueue();
     return $heartbeatAt;
 }