code: public DefaultShardManagerBuilder setCallbackPool(ExecutorService executor, boolean automaticShutdown)
      {
          return setCallbackPoolProvider(executor == null ? null : new ThreadPoolProviderImpl<>(executor, automaticShutdown));
      }
docs: Sets the {@link ExecutorService ExecutorService} that should be used in
  the JDA callback handler which mostly consists of {@link net.dv8tion.jda.core.requests.RestAction RestAction} callbacks.
  By default JDA will use {@link ForkJoinPool#commonPool()}
  <br><b>Only change this pool if you know what you're doing.</b>
  
  @param  executor
  The thread-pool to use for callback handling
  @param  automaticShutdown
  Whether {@link net.dv8tion.jda.core.JDA#shutdown()} should automatically shutdown this pool
  
  @return The DefaultShardManagerBuilder instance. Useful for chaining.