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

[Roman Kennke]

> 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

Looks good to me! Thanks!
Roman