Caching changes

* went back to idea with `/assets/__cb*` path for static assets,
* changed `X-Backend` header to `X-Served-By`,
* added logging for `cachingUtils.filterCachebuster()` middleware.

pinging @aquilax @prein 
https://github.com/Wikia/guidelines/blob/master/JavaScript/CodingConventions.md#objects