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

[Aleksey Shipilev]

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

-- 
Thanks,
-Aleksey