Avoid page_touched update for HTTP GET action=purge requests

Avoid page_touched update for HTTP GET action=purge requests

This will still clear the local DC parser cache and the CDN cache
in all DCs. Therefore, the next page views served by the local
DC will reflect the refreshed content, as will further GET/HEAD
requests by the client that issued the purge using GET/HEAD.

If the problem was imply a stale CDN cache entry, then all
DCs will be up-to-date. If the problem is stale parser cache,
then a proper POST purge request is required to refresh all DCs.

Bug: T92357
Change-Id: I9af12ca8cfff73298f404fd3e2dd4f546621c546