// in it to expire first.
func (c *LRU[K, V]) deleteExpired() {
	c.mu.Lock()
	bucketIdx := c.nextCleanupBucket
	timeToExpire := time.Until(c.buckets[bucketIdx].newestEntry)
	// wait for newest entry to expire before cleanup without holding lock
	if timeToExpire > 0 {
		c.mu.Unlock()
		time.Sleep(timeToExpire)
		c.mu.Lock()
	}
	for _, ent := range c.buckets[bucketIdx].entries {
		c.removeElement(ent)
	}