[go client] Producer::Close() may get stuck

[hrsakai]

Describe the bug
In the situation I confirmed, when entering the logic, C._pulsar_producer_close_async() gets stuck with c<-err in callback and <-c is not executed.
Therefore, Producer::Close() does not finish.

To Reproduce
Steps to reproduce the behavior:

1. Producer connects to broker
2. Revoke the producer's permission to produce(e.g. pulsar-admin namespaces revoke-permission --role "producer.role" public/global/n1)
3. Execute Producer.Close()

I confirmed I can avoid the issue in the following ways.
But I don’t know the best way(Should we modify c++ code?).

1. Use go

func (p *producer) CloseAsync(callback func(error)) {
-	C._pulsar_producer_close_async(p.ptr, savePointer(callback))
+	go C._pulsar_producer_close_async(p.ptr, savePointer(callback))
}

2. Use C.pulsar_producer_close() like Client.Close()

func (p *producer) Close() error {
      res := C.pulsar_producer_close(p.ptr)
      if res != C.pulsar_result_Ok {
              return newError(res, "Failed to close Pulsar client")
      } else {
              return nil
      }
}

[wolfstudy]

The issue has been fixed.