message: Add 'receive' and 'stream' atoms
diff: @@ -179,6 +179,12 @@ using tick_atom = atom_constant<atom("tick")>;
  /// Used for pending out of order messages.
  using pending_atom = atom_constant<atom("pending")>;
  +/// Used as timeout type for `timeout_msg`.
  +using receive_atom = atom_constant<atom("receive")>;
  +
  +/// Used as timeout type for `timeout_msg`.
  +using stream_atom = atom_constant<atom("stream")>;
  +
  } // namespace caf
  namespace std {
  