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).
perhaps make this collection.Map[Int, Int] ? so that it is of immutable type when returned for a given node? (we have access to underlying as a mutable map anyway)