Xrootd failure caching bockelfix

We have been observing substantial failure rates for requests reading in pileup via xrootd at FNAL:

https://savannah.cern.ch/support/?141988

After some debugging we came up with this patch -- which should be overriding the xrootd cache in secondary inputs.  This has been tested via CRAB running a cloned 625 redigi request.  Next step will be hopefully a 62x prerelease to do some scale testing, then if this is OK we'll need this backported to all production releasese...