Merge commit 'da084eb0dfa88493c2718a97c4bfd0e1e935bfce' into MY_master
Mutable ObjectVisitationHelper with a cleaner method TryVisit = IsVisited + Visit
As it turned out none of the recursive algorithms except ApplyChanges has actually needed the advanced visitation strategy (if there exist two ways to reach a certain graph node, then both ones must be walked completely). So with the mutable ObjectVisitationHelper each graph node is visited exactly once.
This should also improve the performance.