Overriding the cache to avoid high xrootd failure rates in pileup dataset reads

This appears to fix a problem with intermittent failures observed in production while reading pileup datasets via xrootd.  FNAL uses local xrootd reads by default and this is starting to happen at the ~10% order.  See:

https://savannah.cern.ch/support/?141988

We need to get this into a 62x prerelease to scale test the fix, then if that is workable will probably need this ported to all production releases.