Multisource xrootd v3

Here's the latest attempt at the new XrdCl.  Compared to before (besides the fact it is again mergable):
1) Ran this with valgrind/helgrind; fixed issues in using the message logger, using the old XrdClient, and some minor lock ordering issues.
2) Try to emulate the older client's behavior - catch readv failures and retry (once) for invalid responses.  I have a ticket open with the Xrootd folks about this - I think it's a server error which is only caught by the new client.
3) Validate the readv vectors before sending them to the remote host.  This hasn't actually caught any issues, but I'd prefer to catch bugs before sending them to the server.

Ran runTheMatrix with no failures, based on the 09/09/2013-1400 IB.  Please verify this before queueing.