func (c *cache) ItemCount() int {
	c.mu.RLock()
	n := len(c.items)
	c.mu.RUnlock()
	return n
}
func (c *cache) ItemCount() int {
	c.mu.RLock()
	n := len(c.items)
	c.mu.RUnlock()
	return n
}
// Returns the number of items in the cache. This may include items that have
// expired, but have not yet been cleaned up.