ReferencePillar ChecksumExtractor has a potential OOM issue

The 'extractAllFileIDs' in the ChecksumExtractor of the ReferencePillar delivers all the file-ids in the ChecksumDatabase in a list.
If the ReferencePillar contains a huge amount of files, then it will cause a OutOfMemory issue.

It should be converted from a List into a Iterator, as done for other database-extractions.

The method is only used by the ReferencePillar (and some tests). Not the ChecksumPillar.