message: Issue Window Prepare Parallelisation
  Really fix stage race condition...
diff: @@ -1510,6 +1510,7 @@ public:
  }
  bool IsSorted() const {
  + lock_guard<mutex> guard(lock);
  return stage == WindowSortStage::SORTED;
  }
  @@ -1519,7 +1520,8 @@ public:
  GlobalSortState &sort_state;
  - mutex lock;
  +private:
  + mutable mutex lock;
  WindowSortStage stage;
  idx_t total_tasks;
  idx_t tasks_assigned;
  