Multisource xrootd v4

This commit switches to the new xrootd client, which is a complete rewrite from the one we use; among other things, it provides an asynchronous API and should be thread-safe.

(There are helgrind warnings, but it mostly seems because helgrind doesn't understand the client's threading mechanisms.)

Additionally, the adaptor is redesigned to balance the load over multiple data servers and dynamically adjust the size of the request according to the server's performance.