func (w *WaterMark) DoneMany(indices []uint64) {
	w.markCh <- mark{index: 0, indices: indices, done: true}
}
func (w *WaterMark) DoneMany(indices []uint64) {
	w.markCh <- mark{index: 0, indices: indices, done: true}
}
// DoneMany works like Done but accepts multiple indices.