add cancel parameter for Spawner::for_each

spawner.h

@@ -24,7 +24,9 @@ struct Spawner {
     void for_each(F&& func) {
         std::lock_guard lock{mutex};
         for (auto& value : values) {
-            func(value);
+            bool cancel = false;
+            func(value, cancel);
+            if (cancel) break;;
         }
     }