message: Add convenience constant invalid_stream_slot
diff: @@ -29,6 +29,9 @@ namespace caf {
  /// connection over IP.
  using stream_slot = uint16_t;
  +/// Identifies an invalid slot.
  +constexpr stream_slot invalid_stream_slot = 0;
  +
  /// Maps two `stream_slot` values into a pair for storing sender and receiver
  /// slot information.
  struct stream_slots : detail::comparable<stream_slots>{
  