NIFI-5225: Purge event data from event repository when Connectable is removed

[FrederikP]

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I introduced the option to purge data from the FlowFileEventRepository (the 5 min ring buffer) to fix this:
https://issues.apache.org/jira/browse/NIFI-5225

And it works for our setup.

[markap14]

@FrederikP all looks good here. I have merged the changes to master. Thanks for the fix!