y/watermark.go
func (w *WaterMark) SetDoneUntil(val uint64) {
	atomic.StoreUint64(&w.doneUntil, val)
}
func (w *WaterMark) SetDoneUntil(val uint64) {
	atomic.StoreUint64(&w.doneUntil, val)
}
// SetDoneUntil sets the maximum index that has the property that all indices
// less than or equal to it are done.