message: move starting new Event to finalize main ht to FinishEvent, so it only starts once the current event is done
diff: @@ -635,7 +635,9 @@ public:
  }
  D_ASSERT(!tasks.empty());
  SetTasks(move(tasks));
  + }
  + void FinishEvent() override {
  //! Now that everything is added to the main ht, we can actually finalize
  auto new_event = make_shared<HashAggregateFinalizeEvent>(op, gstate, pipeline.get(), context);
  this->InsertEvent(move(new_event));
  