Add support for cache-warmers

[Ineluki]

To support cache-warmers, there needs to be a Request-Header that forces a refresh on the called page, e.g.: Fpc-Force-Refresh: 1

To prevent this from becoming an attack surface for DDoS, two options exist:

the value is secret and set in the admin-area (leave free to disable feature)

only calls from a list of IPs can trigger this feature (set in admin area), since the cache warmer will run from a specific server as well.

Edit: replaced URL-parameter with header. I found out that query-parameters are bad, since they are sometimes caught up and cached by shop modules (like layered navigations).