_checkReadOnlyMode = function(req, res, next) {
  if (req.app.locals.redisReadOnly) {
    return res.status(403).json({status: 'FAIL', message: 'read-only mode'});
  }
  next();
}
this express middleware checks if the global readOnly flag for redis connections
 is set and returns an HTTP 403 if found true.
 This middleware should be used on all API routes that modify data inside redis server
 and should not be allowed while redis-commander is startet read-only.

 @param {object} req Express request object
 @param {object} res Express response object
 @param {function} next next Express middleware function to call on success
 @return {*|Promise<any>}
 @private