RFR (S) 8244740: Shenandoah: rename ShenandoahNormalMode to ShenandoahSATBMode

[Roman Kennke]

Ok good!

Thanks,
Roman

> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8244740
> 
> Shenandoah modes define what barriers are emitted. Because of this, "normal" mode should be called
> "satb". It also underlines this mode to be the sibling of "iu" mode.
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8244740/webrev.01/
> 
> Testing: hotspot_gc_shenandoah
>