code: func (db *DB) ZKeyExists(key []byte) (int64, error) {
  	if err := checkKeySize(key); err != nil {
  		return 0, err
  	}
  	sk := db.zEncodeSizeKey(key)
  	v, err := db.bucket.Get(sk)
  	if v != nil && err == nil {
  		return 1, nil
  	}
  	return 0, err
  }
docs: // ZKeyExists checks zset existed or not.