ISSUE-491: Producer block may block forever

[sijie]

Original Issue: apache#491

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Expected behavior

If producer.Close are called,it will close within configured OperationTimeout

Actual behavior

producer.Close may block forever and won't time out after configured OperationTimeout

Steps to reproduce

Just run the perf program for a while and send SIGINT or ctrl+c to it. This situation may be triggered.

This is because the callback of producer.SendAsyc contains a send behavior on a channel which under the same scope with stopCh. When we trigger producer.Close, if case <-stop: happens before case latency := <-ch:, it will block forever because it may be block at here and RequestOnCnx has no time out guarantee.

System configuration

Pulsar version: any