Alternate fallback content for DDoS

[krizhanovsky]

During complex DDoS attack, which can not be reliably filtered out with acceptable confidence that no legitimate users are affected, it has sense to switch to lightweight content of a protected web site. For example if a site has relatively heavy logic requesting a database, then an administrator can define alternate, static and lightweight, site content which is going to be used under DDoS only. The content must be delivered iff:

1. there is a stress condition in sense of HTTP QoS for asymmetric DDoS mitigation #488;
2. a client isn't from the white list;
3. a client doesn't match any limit which clearly define them as a bot.

The alternate content must be loaded to web cache on Tempesta FW start to be always hot. Depends on web server mode.

The site content must be 'locked' the the cache DB, i.e. never evicted #515.