Closing a client using a NullUpdateProcessor crashes instead of closing

[dylanlingelbach]

If I create a Launch Darkly client that uses a NullUpdateProcessor and then I call close on that client, we will crash, because NullUpdateProcessor is currently defined without a close function:

function NullUpdateProcessor() {
  return {
    start: function(callback) {
      setImmediate(callback, null);
    }
  };
}

NullUpdateProcessor should define a no-op close function.