Method is_shadow

include/proxy/http2/Http2DependencyTree.h:96–100

* Added an explicit shadow flag. The original logic * was using an null Http2Stream frame to mark a shadow node * but that would pull in Priority holder nodes too */

   * but that would pull in Priority holder nodes too
   */
  bool
  is_shadow() const
  {
    return shadow == true;
  }

  bool                        active = false;
  bool                        queued = false;