iterator.go
func (m *Nitro) NewIterator(snap *Snapshot) *Iterator {
	if !snap.Open() {
		return nil
	}
	buf := snap.db.store.MakeBuf()
	return &Iterator{
		snap: snap,
		iter: m.store.NewIterator(m.iterCmp, buf),
		buf:  buf,
	}
}
func (m *Nitro) NewIterator(snap *Snapshot) *Iterator {
	if !snap.Open() {
		return nil
	}
	buf := snap.db.store.MakeBuf()
	return &Iterator{
		snap: snap,
		iter: m.store.NewIterator(m.iterCmp, buf),
		buf:  buf,
	}
}
// NewIterator creates an iterator for a Nitro snapshot