Fix the DynamicMultiGroup rebind actions

- Store bucket mappings in an attribute
- Restart feed in the `rebind` method
- Use public class instead of anonymous for function
It's also possible to call `addFeed(FunctionFeed.builder()...)` in `init()`. By calling `addFeed`, it means the feed will be persisted, so it doesn't need to be called in `rebind()`. However, note the existing function feed's `.callable` would not be serializable.