[PR] fix: flush block [pulsar-client-go]

["rfyiamcool (via GitHub)" <gi...@apache.org>]

rfyiamcool opened a new pull request, #1104:
URL: https://github.com/apache/pulsar-client-go/pull/1104

   ### summary
   
   When the producer's close method is executed, the runEventsLoop coroutine will exit. If producer.flush is executed at this time, there is a possibility of blocking.
   
   `If you close first and then flush, `Flush()` will block.`
   
   ```
   go func() {
         close() := func() {
               close(internalQueue)
               producer.Close()
         }
   }()
   
   go func() {
         for _, msg := range internalQueue {
             producer.Send(...)
         }
         producer.Flush() // block
   }
   ```
   
   Sometimes, we cannot control the order of concurrent execution. 😁


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