RFR (S) 8244739: Shenandoah: break superclass dependency on ShenandoahNormalMode

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8244739

Both ShenandoahPassiveMode and ShenandoahIUMode awkwardly subclass ShenandoahNormalMode, only to get
initialize_heuristics() implemented. This would be awkward once we rename ShenandoahNormalMode to
something else.

Webrev:
  https://cr.openjdk.java.net/~shade/8244739/webrev.01/

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey