message: Add scheduled_actor::make_stream_id
diff: @@ -312,6 +312,11 @@ public:
  /// Owning poiner to an `upstream_policy`.
  using upstream_policy_ptr = std::unique_ptr<upstream_policy>;
  + /// Returns a new stream ID.
  + stream_id make_stream_id() {
  + return {ctrl(), new_request_id(message_priority::normal).integer_value()};
  + }
  +
  // Starts a new stream.
  template <class Handle, class... Ts, class Init, class Getter,
  class ClosedPredicate, class ResHandler>
  @@ -339,8 +344,7 @@ public:
  return {stream_id{nullptr, 0}, nullptr};
  }
  // generate new stream ID
  - stream_id sid{ctrl(),
  - new_request_id(message_priority::normal).integer_value()};
  + auto sid = make_stream_id();
  stream<type> token{sid};
  auto ys = std::tuple_cat(std::forward_as_tuple(token), std::move(xs));
  // generate new ID for the final response message and send handshake
  @@ -410,8 +414,7 @@ public:
  rp.deliver(sec::no_downstream_stages_defined);
  return {stream_id{nullptr, 0}, nullptr};
  }
  - stream_id sid{ctrl(),
  - new_request_id(message_priority::normal).integer_value()};
  + auto sid = make_stream_id();
  auto next = stages.back();
  CAF_ASSERT(next != nullptr);
  fwd_stream_handshake<type>(sid, xs);
  