MINIFICPP-1219 - PublishKafka should release connection when stopped

[nghiaxlee]

Thank you for submitting a contribution to Apache NiFi - MiNiFi C++.

In order to streamline the review of the contribution we ask you
to ensure the following steps have been taken:

For all changes:

• Is there a JIRA ticket associated with this PR? Is it referenced
  in the commit message?

• Does your PR title start with MINIFICPP-XXXX where XXXX is the JIRA number you are trying to resolve? Pay particular attention to the hyphen "-" character.

• Has your PR been rebased against the latest commit within the target branch (typically master)?

• Is your initial contribution a single, squashed commit?

For code changes:

• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE file?
• If applicable, have you updated the NOTICE file?

For documentation related changes:

• Have you ensured that format looks appropriate for the output in which it is rendered?

Note:

Please ensure that once the PR is submitted, you check travis-ci for build issues and submit an update to your PR as soon as possible.

[arpadboda]

Mostly looks good, one minor comment.

[szaszm]

The code looks good. Tested minifi on Windows publishing to kafka on linux without TLS: successfully fixed the connection leak on flow reload.
+1

[szaszm]

On Travis, there was an error because the jobs couldn't download rocksdb. Could you trigger Travis again by pushing an empty change? (e.g. empty git amend of the latest commit and force push)

edit: changed my mind. Since all the errors are unrelated, I'm fine with merging right away.

[nghiaxlee]

Thanks for testing and reviewing! We all agree that the errors are unrelated and I think merging will trigger travis anyway and .