Update the FDBIndexRow#ancestorHKey() to no longer need to create a new
Persisitit Key to do a copy into the (Values)HKey. One fewer Key to
process through.
Undo the previous change to PersistitKeyValueStore, but fix the
FDBIndexRow#ancestorHKey() to deal with a potential empty keys. This
occurs in one test (to verify handling the case correctly).
merge tjoneslo: Update the FDBIndexRow#ancestorHKey()

Merge pull request #1580 from tjoneslo/sql-layer/valueshkey-ancestorhkey-for-group-indexes
https://github.com/FoundationDB/sql-layer/pull/1580