Fix send_error_count not reset by amqp_messenger_stop (gh #1386)

[ewertons]

#1386

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• I have read the [contribution guidelines] (https://github.com/Azure/azure-iot-sdk-c/blob/master/.github/CONTRIBUTING.md).
• I added or modified the existing tests to cover the change (we do not allow our test coverage to go down).
• If this is a modification that impacts the behavior of a public API
  • I edited the corresponding document in the devdoc folder and added or modified requirements.
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