code: public final Cache2kBuilder<K, V> loader(AdvancedCacheLoader<K, V> l) {
      config().setAdvancedLoader(wrapCustomizationInstance(l));
      return this;
    }
docs: Enables read through operation and sets a cache loader. Different loader types
  are available: {@link FunctionalCacheLoader}, {@link CacheLoader},
  {@link AdvancedCacheLoader}.
  
  @see CacheLoader for general discussion on cache loaders