message: Make multiplexer accessible through self pointer
diff: @@ -257,6 +257,9 @@ public:
  /// Returns all handles of all `scribe` instances attached to this broker.
  std::vector<connection_handle> connections() const;
  + /// Returns the `multiplexer` running this broker.
  + network::multiplexer& backend();
  +
  protected:
  void init_broker();
  @@ -287,9 +290,6 @@ protected:
  /// @endcond
  - /// Returns the `multiplexer` running this broker.
  - network::multiplexer& backend();
  -
  /// Returns a `scribe` or `doorman` identified by `hdl`.
  template <class Handle>
  auto by_id(Handle hdl) -> optional<decltype(*ptr_of(hdl))> {
  