Improvements

• Change the code of Challenge to 468 to avoid being cached by CDN
• Reduce the probability of deadlock in luigi

Fix

• Mistakenly return 504 error when 502 Bad Gateway
• Access log and error log in site detail do not refresh automatically
• Advanced statistics sometimes have data of 0
• Sites Serving as Static Files mistakenly record some rate-limiting logs caused by 127.0.0.1