func (db *Instance) setIndexID(device, folder []byte, id protocol.IndexID) {
	key := db.indexIDKey(device, folder)
	bs, _ := id.Marshal() // marshalling can't fail
	if err := db.Put(key, bs, nil); err != nil {
		panic("storing index ID: " + err.Error())
	}
}
func (db *Instance) getIndexID(device, folder []byte) protocol.IndexID {
	key := db.indexIDKey(device, folder)
	cur, err := db.Get(key, nil)
	if err != nil {
		return 0
	}

	var id protocol.IndexID
	if err := id.Unmarshal(cur); err != nil {
		return 0
	}

	return id
}