message: blocking_queue::pop_front() returns an rvalue ref
diff: @@ -48,7 +48,7 @@ public:
  non_empty_.notify();
  }
  - T pop_front()
  + T&& pop_front()
  {
  T next;
  @@ -61,12 +61,12 @@ public:
  assert(!queue_.empty());
  // Return the front of the queue
  - next = queue_.front();
  + next = std::move(queue_.front());
  // Remove the object from the queue
  queue_.pop_front();
  }
  - return next;
  + return std::move(next);
  }
  private:
  