RFR (S) 8244732: Shenandoah: move heuristics code to gc/shenandoah/heuristics

[Roman Kennke]

Ok!

Thanks,
Roman

> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8244732
> 
> For cleanliness reasons, it makes sense to move all related heuristics code to
> gc/shenandoah/heuristics subfolder. This mostly affects the move of shenandoahHeuristics.hpp itself.
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8244732/webrev.01/
> 
> Testing: hotspot_gc_shenandoah
>