NIFI-3039 Provenance Repository - Fix PurgeOldEvent and Rollover Size Limits (0.x)

[jskora]

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NIFI-3039 Provenance Repository - Fix PurgeOldEvent and Rollover Size Limits

Added low water mark for purgeOldEvents() to prevent thrashing on event cleanup.
Adjusted rollover high water mark to match purgeOldEvents() to prevent overrunning "nifi.provenance.repository.max.storage.size".
Moved high/low water marks to constants.
Adjusted looping logic in mergeJournals() to use ".firstKey()" instead of ".entrySet().iterator().next()" to avoid unnecessary object creation.

[joewitt]

@jskora This is our oldest untouched PR. Do you still think it is good to go?

@markap14 Can you take a look?

[jskora]

@joewitt Since 0.x is end-of-life, this is OBE and should be closed.

IIRC, the root problem was writing records first and then checking afterward to see if storage limits were exceeded and a rollover needed. If the storage limits don't leave a margin before reaching the physical capacity, writes can fill the disk and crash the NiFi and possible the system. I looked at the 1.x repositories in depth since they were re-written, but if the same write-then-check behavior exists this could still be a problem.

[mosermw]

@jskora I think you are the only one who can close this PR. Would you mind? Thanks!

[jskora]

Overcome by events.