fix: flush block

[rfyiamcool]

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. 😁

[tisonkun]

This patch doesn't "fix" an issue if any because it's possible that the check passed first but the producer gets closed just after the check passed.