Title: Configurable list of trusted proxies to handle x-forwarded-for header

Description:
It'd be helpful for improving security to handle x-forwarded-for header from the trusted proxies only if they are specified to filter out forged XFF header from malicious clients.
And add a configuration parameter to specify such trusted proxies. List of CIDR, for example.
In a similar way as set_real_ip_from option in Nginx.