code: func NewThumbMeta(kv sorted.KeyValue) *ThumbMeta {
  	return &ThumbMeta{
  		mem: lru.New(memLRUSize),
  		kv:  kv,
  	}
  }
docs: // NewThumbMeta returns a new in-memory ThumbMeta, backed with the
  // optional kv.
  // If kv is nil, key/value pairs are stored in memory only.