uint64_t max_age() const { return max_age_; }
  size_t max_entries() const { return max_entries_; }

 private:
  struct Entry {
    /// The timestamp (seconds) at which the entry was added to the cache.
    uint64_t timestamp;

    /// The entry's value.
    T value;

    /// A list iterator pointing to the entry's position in the LRU list.
    std::list<std::string>::iterator lru_iterator;

  ///
  /// A file block consists of the block data, the block's current position in
  /// the LRU cache, the timestamp (seconds since epoch) at which the block
  /// was cached, a coordination lock, and state & condition variables.
  ///
  /// Thread safety:
  /// The iterator and timestamp fields should only be accessed while holding
  /// the block-cache-wide mu_ instance variable. The state variable should only