Performance improvements and Refactorings for FossilDbIterators

[fm3]

VersionedFossilDbIterator and its dependents are used when multiple keys of fossildb-saved data are processed sequentially. Some ideas how to improve them:

• Cleanup: Generic Reversion-Aware Iterator? Build on top of VersionedFossilDbIterator? Could replace VersionedAgglomerateToGraphIterator, VersionedSegmentToAgglomerateChunkIterator and VolumeTracingBucketProvider?
• For proofreading iterators, save perf by skipping fetching content when only version is needed in iterator (different from volume, because here we don’t need to update the segment index)
  • that might need new fossildb api (ListKeysWithVersions)
• Make the iterators async? Use async fossildb api?

Should be tackled after #7839 is merged