Implement timeouts for the Xrootd client.

The default timeout is now set to 10 minutes; previously, it varied
between 3 minutes and 8 hours, depending on the operation.  Further,
the timeout can now be set by the CMSSW pset and the site local
config service.

It appears that timeouts were never set for fallback files;
this issue is now fixed.
Why is this static? It only seems accessible via member functions.