code: function getPayload(req, res, next) {
    req.hasCallback = false;
    req.payload = req.body.payload || [];
  
    // Determines if the request is asynchronous or not
    req.payload.forEach(function(arg, i) {
      if (arg === '__clientCallback__') {
        req.hasCallback = true;
        req.clientCallbackIndex = i;
      }
    });
  
    debug('Got payload' + (req.hasCallback ? ' with callback' : '') + ': ' +
      JSON.stringify(req.payload, null, 3));
  
    next();
  }
docs: Processes the client-side payload, and transforms the client-side
  callback function signal to an actual callback function