Move replacement of Cache-Control header from backend to frontend

Varnish frontends and backends can be treated equally, as both
receive purges. Frontends should replace/change the Cache-Control
header to control caching by clients / shared caches, but until
then Varnish should use the Cache-Control header sent by MediaWiki.
This also allows for more tiers of caching (esams etc).

Change-Id: I49630ff568a5c7b93586d6f45c4f8634bc17949a