Sorting is now done using a sorting index rather than loading the entire

Sorting is now done using a sorting index rather than loading the entire
result set into memory and doing a merge sort.  The old merge sort technique
was a carry-over from SQLite version 1.  The new method uses a bounded amount
of memory and scales to much larger result sets.  There are still errors:
some 39 regression tests fail. (CVS 2653)

FossilOrigin-Name: 09db0a24241f9248584250d1728117b8a3159626