code: func (kv *keyValue) Delete(key string) error {
  	kv.mu.Lock()
  	defer kv.mu.Unlock()
  	err := kv.db.Remove(&bson.M{mgoKey: key})
  	if err == mgo.ErrNotFound {
  		return nil
  	}
  	return err
  }
docs: // Delete removes the document with the matching key.