Introduce New Default Scan Ordering 'None'

Introduce a new default scan ordering type 'none'.
In gencon configuration, scan ordering will be set
to hierarchical if it has not already been overidden
by an option. In balanced, scan ordering will be set to
dynamic BF if it has not already been overidden by an
option.

Signed-off-by: Jonathan Oommen <jon.oommen@gmail.com>

gc/base/Configuration.cpp

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 1991, 2020 IBM Corp. and others
+ * Copyright (c) 1991, 2021 IBM Corp. and others
  *
  * This program and the accompanying materials are made available under
  * the terms of the Eclipse Public License 2.0 which accompanies this
@@ -473,6 +473,9 @@ MM_Configuration::initializeGCParameters(MM_EnvironmentBase* env)
 #if defined(OMR_GC_MODRON_SCAVENGER)
 		if (extensions->scavengerEnabled) {
 			extensions->splitFreeListSplitAmount = (extensions->gcThreadCount - 1) / 8  +  1;
+			if (MM_GCExtensionsBase::OMR_GC_SCAVENGER_SCANORDERING_NONE == extensions->scavengerScanOrdering) {
+				extensions->scavengerScanOrdering = MM_GCExtensionsBase::OMR_GC_SCAVENGER_SCANORDERING_HIERARCHICAL;
+			}
 		} else
 #endif /* OMR_GC_MODRON_SCAVENGER */
 		{

gc/base/GCExtensionsBase.hpp

@@ -444,7 +444,8 @@ class MM_GCExtensionsBase : public MM_BaseVirtual {
 
 #if defined(OMR_GC_MODRON_SCAVENGER) || defined(OMR_GC_VLHGC)
 	enum ScavengerScanOrdering {
-		OMR_GC_SCAVENGER_SCANORDERING_BREADTH_FIRST = 0,
+		OMR_GC_SCAVENGER_SCANORDERING_NONE = 0,
+		OMR_GC_SCAVENGER_SCANORDERING_BREADTH_FIRST,
 		OMR_GC_SCAVENGER_SCANORDERING_DYNAMIC_BREADTH_FIRST,
 		OMR_GC_SCAVENGER_SCANORDERING_HIERARCHICAL,
 	};
@@ -1542,7 +1543,7 @@ class MM_GCExtensionsBase : public MM_BaseVirtual {
 		, gcThreadCountForced(false)
 		, dispatcherHybridNotifyThreadBound(16)
 #if defined(OMR_GC_MODRON_SCAVENGER) || defined(OMR_GC_VLHGC)
-		, scavengerScanOrdering(OMR_GC_SCAVENGER_SCANORDERING_HIERARCHICAL)
+		, scavengerScanOrdering(OMR_GC_SCAVENGER_SCANORDERING_NONE)
 		/* Start of options relating to dynamicBreadthFirstScanOrdering */
 		, gcCountBetweenHotFieldSort(1)
 		, gcCountBetweenHotFieldSortMax(6)