message: Add abstract_downstream::paths
diff: @@ -162,10 +162,15 @@ public:
  /// Returns how many items should be stored on individual paths in order to
  /// minimize latency between received demand and sent batches.
  - long min_buffer_size() const {
  + inline long min_buffer_size() const {
  return min_buffer_size_;
  }
  + /// Returns all currently available paths on this downstream.
  + const path_list& paths() const {
  + return paths_;
  + }
  +
  protected:
  void send_batch(downstream_path& dest, long chunk_size, message chunk);
  