code: public final Cache2kBuilder<K, V> manager(CacheManager manager) {
      if (this.manager != null) {
        throw new IllegalStateException("manager() must be first operation on builder.");
      }
      this.manager = manager;
      return this;
    }
docs: The manager, the created cache will belong to. If this is set, it must be the
  first method called.
  
  @param manager The manager the created cache should belong to,
  or {@code null} if the default cache manager should be used
  @throws IllegalStateException if the manager is not provided immediately after the builder is created.