finish the cleanup of the netcard list container

finish the cleanup of the netcard list container

- move it from global to the engine
- use a std::unordered_map instead of a xbt_dict. Its order being
  harder to predict, this forces to sort the list in the routing
  integration tests.

I'm not completely happy about the register/unregister functions being
public, but I cannot find anything better, and it's later already...