Verify 2.8 freelist is upgrade-downgrade safe with 2.6

Know issues:

• If compact is run on a downgraded server (ran 2.8, now running 2.6), it will attempt to orphan the deleted list, but will not orphan the 2.8-only portion. If such a server was upgraded in the future, it would try to use deleted records in freed extents.
  • Solution: have 2.6 zero-out reserved bytes in NamespaceDetails when orphaning the deleted list during a compact.