public ConnectionFactoryBuilder setListenerExecutorService(ExecutorService executorService) {
    this.executorService = executorService;
    return this;
  }
Set a custom {@link ExecutorService} to execute the listener callbacks.

Note that if a custom {@link ExecutorService} is passed in, it also needs to be properly
shut down by the caller. The library itself treats it as a outside managed resource.
Therefore, also make sure to not shut it down before all instances that use it are
shut down.

@param executorService the ExecutorService to use.
src/main/java/net/spy/memcached/ConnectionFactoryBuilder.java