code: func NewLowlevel(db *leveldb.DB, location string) *Lowlevel {
  	return &Lowlevel{
  		DB:        db,
  		location:  location,
  		folderIdx: newSmallIndex(db, []byte{KeyTypeFolderIdx}),
  		deviceIdx: newSmallIndex(db, []byte{KeyTypeDeviceIdx}),
  		closeMut:  &sync.RWMutex{},
  	}
  }
docs: // NewLowlevel wraps the given *leveldb.DB into a *lowlevel