message: Add `invalid_stream` for initializing stream<T>
diff: namespace caf {
  +struct invalid_stream_t {};
  +
  /// Identifies an unbound sequence of messages.
  template <class T>
  class stream {
  @@ -39,6 +41,10 @@ public:
  // nop
  }
  + stream(invalid_stream_t) {
  + // nop
  + }
  +
  inline const stream_id& id() const {
  return id_;
  }
  @@ -52,6 +58,8 @@ private:
  stream_id id_;
  };
  +constexpr invalid_stream_t invalid_stream = invalid_stream_t{};
  +
  } // namespace caf
  #endif // CAF_STREAM_HPP
  