BAC-550: move "served by" info to header

* remove ``<!-- Served by ... in ... sec -->`` comment (introduce ``$wgDisableReportTime`` global variable)
* add ``X-Backend-Response-Time`` response header with response generation time in ms
* add ``X-Served-By`` header with hostname of apache machine that generated the response (will be appended with Varnish machines handling client response)

@michalroszka / @prein / @Wikia/platform-team 


```
$ curl -I "http://poznan.macbre.wikia-dev.com/wiki/Gzik" 
HTTP/1.1 200 OK
X-Backend-Response-Time: 0.852
X-Served-By: dev-macbre, cache-s24-SJC, cache-fra1227-FRA
```