journal-file: when individual hash chains grow too large, rotate

Even with the new keyed hash table journal feature: if an attacker
manages to get access to the journal file id it could synthesize records
that result in hash collisions. Let's rotate automatically when we
notice that, so that a new journal file ID is generated, our performance
is restored and the attacker has to guess a new file ID before being
able to trigger the issue again.

That said, untrusted peers should never get access to journal files in
the first case...

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