code: func (b *FastLogStore) GetUint64(key []byte) (uint64, error) {
  	val, err := b.Get(key)
  	if err != nil {
  		return 0, err
  	}
  	if len(val) != 8 {
  		return 0, errors.New("invalid number")
  	}
  	return binary.LittleEndian.Uint64(val), nil
  }
docs: // GetUint64 is like Get, but handles uint64 values