RFR (S) 8220350: Refactor ShenandoahHeap::initialize

[Aleksey Shipilev]

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

Fix:
  http://cr.openjdk.java.net/~shade/8220350/webrev.01/

This prepares Shenandoah code for accepting bugfixes in this method. Some asserts are added to
verify important fields are indeed initialized. Pacer cache moved out of ShHeapRegion to resolve
initialization circularity -- it does not matter anyway, because increase_live_data_gc_words is
called rarely itself, being protected by the cache during mark.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey