HeapRegionNode and ReferenceEdge both have had the equals() and hashCode()

HeapRegionNode and ReferenceEdge both have had the equals() and hashCode()
methods updated to exclude transient attributes like reachability information.
If these objects are put into work sets and those attributes are changed they
have become different objects, according to the hash set.  So equals() was changed
and other methods are used for determining equality in other situations, like
asking whether heap region nodes in different graphs are equivalent.

Also, a few other minor changes.