Varnish caching backports to release-208

* BAC-1252: remove ActivityFeed purge request on edit
* BAC-1262: Purge only enabled namespaces and purge them with correct prefix
* BAC-1254: fix X-Cache header
* BAC-1258:  Add sFlow monitoring for purges from MW

@prein / @michalroszka / @hakubo