Include Cache-Control support for XrdHttp to invalidate cached data

[jthiltges]

Implementing Cache-Control: no-cache for an HTTP client to trigger cache invalidation would be a handy enhancement.

Currently, to monitor that xcaches can reach an origin server, we've needed to deploy cronjobs to remove cache entries of test files. Being able to force a cache to forward the request to the origin would be helpful.

In addition, this might help with allowing cvmfs clients to retrieve updated content via xcache. Glancing at the cvmfs code, it seems to retry with no-cache if there's a mismatch.