MINIFICPP-996 - Output port keeps printing info message implying tran…

[arpadboda]

…smission

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]

This case cannot occur at the moment as NiFi handles this improperly.
When fixed and tested in scope of apache/nifi#3647 I found that we throw exception and execute rollback in case 200 is received, even though it's a valid case, 200 is not an error.
Transaction being null is for error cases, so this function now creates a dummy transaction, which then gets removed and the happy path is executed. (transfer was successful)

[bakaid]

Generally looks good. If the protocol headers sent only contain the maximum supported protocol version, then it's ok (otherwise we would lose compatibility with older NiFis).
As far as I see we don't actually use the negotiated protocol version, we ensure compatibility with both versions at the same time. This is a good solution for now, but in the future there might be a need for knowing the exact version.
AppVeyor build is failed because of a timeout, so please rebase this to master and let's hope it builds now.
Also, I am not sure whether we have working S2S tests on Windows now, if not, a Windows manual test would be warranted.