Experiment with FTS

[patrick-austin]

• Test for multiple files, trying to determine FTS's soft limit (caused by large JSON with the request). Expect this at ~1000 according to Tom Byrne. Latest testing gives between 2000 and 4000 - presumably this depends on the actual length of the filenames etc?
• Test use of xrd python client to see how difficult/performant it would be to read metadata from files on the IDC
• Test use of direct CephFS mount to see how difficult/performant it would be to read metadata from files on the IDC
• Test multisource jobs and document how exactly they work, and whether it would be suitable to use them for avoiding repeated requests for the same file resulting in staging from tape
• Test checksums at all endpoints