code: public final Cache2kBuilder<K,V> prefetchExecutor(Executor v) {
      config().setPrefetchExecutor(new CustomizationReferenceSupplier<Executor>(v));
      return this;
    }
docs: Thread pool / executor service to use for refresh ahead and prefetch operations. If not specified the
  same refresh ahead operation will use the thread pool defined by {@link #loaderExecutor(Executor)}
  or a cache local pool is created.
  
  @see #loaderThreadCount(int)
  @see #loaderExecutor(Executor)