MINIFICPP-2291 Fix site-to-site transfer of large files

[fgerlits]

Previously, the site-to-site client treated the TRANSACTION_FINISHED_BUT_DESTINATION_FULL message from the server as a failure, and kept rolling back and retrying the flow file.

Now, we treat it as success but add a yield so the server has time to clear the incoming connection, the same way as NiFi does.

https://issues.apache.org/jira/browse/MINIFICPP-2287
https://issues.apache.org/jira/browse/MINIFICPP-2291

I have also included the fix of two unrelated bugs I noticed during testing; these are too small to merit their own pull requests.

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