In MySQL 5.6, QUICK_SELECT_DESC calls handler->set_end_range() to
inform the storage engine about the bounds of the range being scanned.

MariaDB doesn't have that (handler::set_end_range call was back-ported
but it is not called from QUICK_SELECT_DESC).
Instead, it got prepare_*scan() methods from TokuDB.

Implement these methods so that MyRocks has information about the range
end.