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).
DepthTracker trait functionality seems to me to be a subset of PathLengthTracker where depthTracker equals nextvisitDistanceTracker ?. Can we just delete it and use PathLengthTracker instead?