message: Add missing forward declarations, clean up header
diff: namespace caf {
  namespace net {
  -class multiplexer;
  -class socket_manager;
  +// -- templates ----------------------------------------------------------------
  +
  template <class Application, class IdType = unit_t>
  class transport_worker;
  +
  template <class Application, class IdType = unit_t>
  class transport_worker_dispatcher;
  +// -- classes ------------------------------------------------------------------
  +
  +class endpoint_manager;
  +class multiplexer;
  +class socket_manager;
  +
  +// -- structs ------------------------------------------------------------------
  +
  struct network_socket;
  struct pipe_socket;
  struct socket;
  struct stream_socket;
  -struct tcp_stream_socket;
  struct tcp_accept_socket;
  +struct tcp_stream_socket;
  -using socket_manager_ptr = intrusive_ptr<socket_manager>;
  +// -- smart pointers -----------------------------------------------------------
  +using endpoint_manager_ptr = intrusive_ptr<endpoint_manager>;
  using multiplexer_ptr = std::shared_ptr<multiplexer>;
  -
  +using socket_manager_ptr = intrusive_ptr<socket_manager>;
  using weak_multiplexer_ptr = std::weak_ptr<multiplexer>;
  } // namespace net
  