Optimize probing

[ilyaraz]

In particular, try radix sort instead of std::sort.

[ludwigschmidt]

Just to clarify: I guess this is for the post-probing sorting of the candidates, not the sorting in the CP hash (where we use the incremental sorter)? It would also be interesting to see whether the sorting helps at all actually. Since the DataStorage class has a prefetcher built in, the advantage of sorting might not be that large.