Adding specific sort function for FT hits for PrStoreFTHits

It's not for a functional requirement, just a try for perf, ft hits are currently sorted (here) with ranges::sort by ids first, and by hits.

Ids are already sorted, they are just with this arrangement -> 1111...0000...3333...2222... etc

So I wanted to try to take advantage from the knowledge of the input structure for this sort by swaping range of ids and to std::sort only hits for each of these ranges. The difference between these two sorts seems to be around 20% according to valgrind

left: after, right: before