message: core: added comment to explain guard
diff: @@ -30,6 +30,8 @@ public:
  iterator erase(iterator it) { return _queue.erase(it); }
  + // This guard serves the purpose to combine a get_front with a pop_front.
  + // Thus, no one can interfere between the two steps.
  class Guard {
  public:
  Guard(LockedQueue& locked_queue) : _locked_queue(locked_queue)
  