Filesort currently uses either radixsort or quicksort when sorting records.
Radixsort is very fast, but only for certain data sets (short keys).
For *most* cases when my_qsort2() is used, it turns out that std::stable_sort()
is faster.

This worklog will extend the choice of sorting algorithms, preferring
std::stable_sort() when that is assumed to be better than the other two.