QtHelp: Don't cache query rows when reading the index table

When registering documentation, QHelpDBReader::indexTable() copies
the index, file and contents tables of the .qch file into memory.
The query cached all the fetched rows on top of that, holding another
copy of the same data.

Make the queries forward-only, so that the fetched rows aren't cached.

Task-number: QTBUG-150684
Pick-to: 6.12 6.8
Change-Id: Ic94c45d6296b8137c825a62331061d78a6e3a688
Reviewed-by: Kai Köhne <kai.koehne@qt.io>