ISSUE-584: Duplicate producer when create producer timeout

[sijie]

Original Issue: apache#584

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When the broker or zookeeper is temporary offline, which will cause the producer timeout and re-connect to the broker. But if the broker has already done the previous create producer request, the reconnection will fail with an "already connected" error.

apache/pulsar#5571 has addressed this issue in Java client, but go client still encounter this issue.

apache#582 add epoch to go client to solve this issue.