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).
there is duplication of code with QueueBasedTraverser.next() -- only findNextToVisit() is new here. Perhaps make 'protected def findNextToVisit() = { getExisting.... }' in QueueBasedTraverser and override it here?