Change EdgeManagerPlugin APIs to return an abstracted Range rather than a list of integers

EdgeManagers end up running the following loops.

for (int i=0; i<getContext().getVertexNumTasks(getContext().getVertexName()); ++i) {
      pendingTasks.add(Integer.valueOf(i));
    }

Instead, this should be a BitSet or abstracted into a Range interface which can be used to represent this data more efficiently.