RFR 8223774: Shenandoah: Refactor ShenandoahRootProcessor and family

[Roman Kennke]

>> *) shenandoahArguments.cpp: this block is better be in
>> ShenandoahTraversalHeuristics?
>>
>>   186   if (strcmp(ShenandoahGCHeuristics, "traversal") == 0) {
>>   187     FLAG_SET_DEFAULT(ShenandoahConcurrentScanCodeRoots, false);
>>   188   }
>>   189
> 
> We don't have this heuristic, let's do it in follow up RFE.

We do:
src/hotspot/share/gc/shenandoah/heuristics/shenandoahTraversalHeuristics.hpp

and

src/hotspot/share/gc/shenandoah/heuristics/shenandoahTraversalHeuristics.cpp

Roman