Use XrdCl::QueryCode::Head to retrieve cache control values

[alja]

@bbockelm @osschar

Do not merge this PR!
The new query code for cache control parameters is in my local repository.
The change is dependent to alja/xrootd#89

[alja]

The xrootd-related development has been moved to PR xrootd/xrootd#2586

[bbockelm]

@alja - there's still a lot of merge conflicts here ... is the branch heading to completion?

[osschar]

Andy is still shuffling things around a bit in pss/posix ... but I think the XrdCl part is mostly fixed.
This is dependent on changes going into xrootd and we're figuring out if this can go into 5.9 or we have to wait until 6. I mentioned to Andy you guys will probably pick it all up anyway as you tightly control the deployment and don't have old plugins to think about.

[alja]

@bbockelm The related ETag validation changes in xrootd have been merged to the main branch. I will test the PR again with the main branch in this repository.

[alja]

The changes are tested with the main xrootd branch.