Duplicate producer when create producer timeout

[freeznet]

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.

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

[cckellogg]

Addressed in #582