Update the FDBIndexRow#ancestorHKey() 

No longer need to create a new Persisitit Key to do a copy into the (Values)HKey. One fewer Key to
process through.

Remove the comment in ValuesHKey#ancestorHKey about the key shortening being wrong: https://trello.com/c/p9EBACdv

The change in PersistitKeyValueSource is to allow the FDBIndexRow test, which creates an empty key for one test, to pass correctly.