Function continual_task_sink 

fn continual_task_sink<'scope>(
    name: &str,
    to_append: VecCollection<'scope, Timestamp, SourceData, Diff>,
    append_times: VecCollection<'scope, Timestamp, (), Diff>,
    as_of: Antichain<Timestamp>,
    write_handle: impl Future<Output = WriteHandle<SourceData, (), Timestamp, StorageDiff>> + Send + 'static,
    start_signal: StartSignal,
    output_frontier: Rc<RefCell<Antichain<Timestamp>>>,
) -> Button