Google Code-in

In the referenced bug Alexander suggests a couple of changes. This task is to partially adopt one of his changes: implement a Scheduler based on priority_queue, but maintain support for Remove.

1. Apply these two patches. It won't build yet.
2. Create an inner class PriorityQueueScheduler::PriorityQueue to implement remove, as sketched in this SO post
3. Use your new inner class instead of the plain priority_queue, and implement Remove() as required by the Scheduler base class.
4. Now check that ns-3 still builds and all tests pass.