producer not stop after shutdown since #3199

[yuz10]

BUG REPORT

1. Please describe the issue you observed:

• What did you do (The steps to reproduce)?

   public static void main(String[] args) throws MQClientException, InterruptedException {

        DefaultMQProducer producer = new DefaultMQProducer("ProducerGroupName");
        producer.setNamesrvAddr("localhost:9876");
        producer.start();
        producer.shutdown();
    }

• What did you expect to see?
  the producer will exit.

• What did you see instead?
  the producer dont exit forever.

2. Please tell us about your environment:

3. Other information (e.g. detailed explanation, logs, related issues, suggestions how to fix, etc):

[panzhi33]

I was fixed，can see #3453

[yuz10]

I was fixed，can see #3453

See, I will close this.