RFR (S) 8242075: Shenandoah: rename ShenandoahHeapRegionSize flag

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8242075

There are ShenandoahMinRegionSize and ShenandoahMaxRegionSize flags. Having
Shenandoah*Heap*RegionSize that overrides them is not very clear. Let's rename it to
ShenandoahRegionSize.

Webrev:
 https://cr.openjdk.java.net/~shade/8242075/webrev.01/

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey