Improved traversers

Added abstract class QueueBasedTraverser holds general logic for most
traversers.
Fixed errors in InfoKeepers.
Added InfoKeeper that uses IntToBoolean fastutil map to store Boolean
values.
Changed BreadthFirstTraverser so that first node is the node we start
traverse from (to be consistent with other traversals).
sorry, still don't understand. so it is to keep track of last parent node that adds the node to the queue? how can multiple parent nodes add the same node to the queue in our onlyOnce scenario ?