NIFI-6559 FlowFile Repo Journal Recovery Should not Fail if External …

[patricker]

…Overflow Files are Missing

Description of PR

When NiFi is journaling the FlowFile repository changes to disk it sometimes writes Overflow files if it exceeds a certain memory threshold.

These files are tracked inside of the *.journal files as External File References. If one of these external file references is deleted or lost the entire journal fails to recover.

Instead, I feel this should work more like FlowFile's that lose their queue, or Content in the Content Repository that has lost it's FlowFile. Log it, and move on.

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[patricker]

After JIRA discussion, cancelling work.