message: Prefer const stream<T> refs in scheduled_actor
diff: @@ -406,7 +406,7 @@ public:
  /// Adds a stream stage to this actor.
  template <class In, class... Ts, class Init, class Fun, class Cleanup>
  stream<typename stream_stage_trait_t<Fun>::output>
  - add_stage(stream<In>& in, std::tuple<Ts...> xs,
  + add_stage(const stream<In>& in, std::tuple<Ts...> xs,
  Init init, Fun fun, Cleanup cleanup) {
  CAF_ASSERT(current_mailbox_element() != nullptr);
  using output_type = typename stream_stage_trait_t<Fun>::output;
  @@ -435,7 +435,7 @@ public:
  /// Adds a stream stage to this actor.
  template <class In, class Init, class Fun, class Cleanup>
  stream<typename stream_stage_trait_t<Fun>::output>
  - add_stage(stream<In>& in, Init init, Fun fun, Cleanup cleanup) {
  + add_stage(const stream<In>& in, Init init, Fun fun, Cleanup cleanup) {
  return add_stage(in, std::make_tuple(), std::move(init),
  std::move(fun), std::move(cleanup));
  }
  @@ -456,7 +456,7 @@ public:
  /// Adds a stream sink to this actor.
  template <class In, class Init, class Fun, class Finalize>
  result<typename stream_sink_trait_t<Fun, Finalize>::output>
  - add_sink(stream<In>& in, Init init, Fun fun, Finalize finalize) {
  + add_sink(const stream<In>& in, Init init, Fun fun, Finalize finalize) {
  CAF_ASSERT(current_mailbox_element() != nullptr);
  delegated<typename stream_sink_trait_t<Fun, Finalize>::output> dummy_res;
  //using output_type = typename stream_sink_trait_t<Fun, Finalize>::output;
  