NIFI-6853 - flowFileEvent combineCounters hashmap overwritten

[ivan1221]

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Enables X functionality; fixes bug NIFI-YYYY.

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[ivan1221]

Fix this issue:

https://issues.apache.org/jira/browse/NIFI-6853

[ivan1221]

Added unit test to verify behavior of contribution

[markap14]

Thanks for catching/fixing this @ivan1221! +1 merged to master.