Generates a cache key identifying the provided HTTP request and simillar
requests that may share the same page content.

@param {Express.Request} request The current express.js HTTP request.
@return {string} The generated cache key.
function defaultKeyGenerator(request) {
  let protocol = request.protocol;
  let host = request.get('Host');
  let url = request.originalUrl;

  return protocol + ':' + host + url;
}