y/watermark.go
func (w *WaterMark) Begin(index uint64) {
	atomic.StoreUint64(&w.lastIndex, index)
	w.markCh <- mark{index: index, done: false}
}
func (w *WaterMark) Begin(index uint64) {
	atomic.StoreUint64(&w.lastIndex, index)
	w.markCh <- mark{index: index, done: false}
}
// Begin sets the last index to the given value.