[12u] RFR (S) 8220350: Refactor ShenandoahHeap::initialize

[Aleksey Shipilev]

Hi,

Please review the backport for Shenandoah change that allows to unblock follow-up functionality fixes.

Original RFE:
 https://bugs.openjdk.java.net/browse/JDK-8220350

Original change:
  http://cr.openjdk.java.net/~shade/8220350/webrev.02/

Patch does not apply cleanly, because there are minor changes in SATBQueue interface and whitespace
difference. 12u change:
  http://cr.openjdk.java.net/~shade/8220350/webrev.12u.02/

Testing: hotspot_gc_shenandoah, eyeballing the differences against original change

-Aleksey