message: Add forward declarations for option_* classes
diff: @@ -82,6 +82,9 @@ class actor_system;
  class actor_system_config;
  class behavior;
  class blocking_actor;
  +class config_option;
  +class config_option_set;
  +class config_value;
  class deserializer;
  class downstream_manager;
  class downstream_manager_base;
  