Use XRootD if possible and process files in parallel

Modifies lbninstall to:

• Try to copy using xrdcp if possible, falls back to HTTP if that fails (requires credentials unfortunately)
• Download and unpack all files in parallel. Using joblib for now, but I could use something else if preferred?

This makes installations on CVMFS of large projects a lot faster. I'll open a JIRA task shortly with more thoughts about making installations faster.

I'm leaving this as WIP for now to see if this is an avenue worth pursuing.