Varnish caching backports to release-207

Backports of #2671, #2667 and #2666 

* BAC-1251: handle If-None-Match and disable the support of If-Modified-Since
* BAC-1250: wgInvalidateCacheOnLocalSettingsChange - set to false
* BAC-1245: do not add IP to Tracker beacon URL

@prein / @michalroszka / @Wikia/platform-team