Fix resurrection by hanging ref and deletion by invalidated ancestry

This PR

Does not trust the ancestors of any stats file written before this version
- A VALID_ANCESTORS component is added as a flag from this version onward
- For older stats files, treated as no ancestors
Skips SSTable renumbering when assumeCfIsEmpty
When SSTables need to be renumbered, the stats file gets deleted
Adds palantir_cassandra.dry_run_ancestor_deletion system parameter to only dry-run deleting leftover compaction ancestors.

palantir / cassandra