code: func (c *Cache) RemoveOldest() (key string, value interface{}) {
  	if !c.nolock {
  		c.mu.Lock()
  		defer c.mu.Unlock()
  	}
  	return c.removeOldest()
  }
docs: // RemoveOldest removes the oldest item in the cache and returns its key and value.
  // If the cache is empty, the empty string and nil are returned.