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.

Whenever a ReadV error occurs, raise a CMSSW exception; do not
allow this to be cast down to a ROOT warning.  Exception information
is improved so the last Xrootd server message (or timeouts) are
recorded.

Finally, this fixes an older bug in XrdReadv.cc which causes the
transaction timeout to not be reset at the beginning of ReadV calls.
This causes the XrdAdaptor to throw an exception on ReadV
calls if it has been more than the timeout (default, 8 hours) between
Read calls (which did reset the transaction timeout).

This is a backport of PR #5714 per request from @davidlange6