message: [c++ grpc] Make wait_callback final
diff: @@ -26,7 +26,7 @@ namespace bond { namespace ext { namespace gRPC {
  /// been called. Then, the status() and response() member functions can be
  /// called to inspect the results.
  template <typename TResponse>
  -class wait_callback
  +class wait_callback final
  {
  public:
  wait_callback() : _impl(std::make_shared<impl>()) { }
  @@ -95,7 +95,7 @@ private:
  /// The interesting guts of wait_callback. We use an impl class so that
  /// wait_callback can be copied and all the copies affect the same underlying
  /// state.
  - struct impl
  + struct impl final
  {
  impl() = default;
  impl(const impl&) = delete;
  