NIFI-2776: When joining a cluster, if a processor is stopping but clu…

[markap14]

…ster indicates that processor should be running, cause processor to start when its last thread finishes

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[joewitt]

Please rebase to latest master so we can get better testing/contrib-check.

Also, please convert those unit tests to integration tests. They are very timing sensitive and highly reflective of the types of things we just coverted to IT in many cases to get the builds stable again.