RFR (S) 8244737: Shenandoah: move mode code to gc/shenandoah/mode

[Roman Kennke]

Ok!

Thanks,
Roman

> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8244737
> 
> Similarly to JDK-8244732, for cleanliness reasons, it makes sense to move all related mode code to
> gc/shenandoah/mode subfolder.
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8244737/webrev.01/
> 
> Testing: hotspot_gc_shenandoah
>