Write and deploy tests of simultaneous FTS transfers into GSI-ROOT

Recently GSI-ROOT saw some issues coping with multiple simultaneous transfers onto its single disk. Plans to move to a lustre filesystem are currently moving slowly, so another option could be to extend to three disks rather than one. We want to test before and after this change to see (1) if the issue is reproducible and independent of the RSE data volume limit (which was previously reached), and (2) if upgrading to 3 disks solves the problem. 

Outcomes: test scripts in the shared git repo and results attached to this ticket and presented in the WP2 workshop 9/10 December.

It is not necessary to perform the upgrade before the second testing day "FDR2", but we should identify what I/O requirements we should ask of RSEs in the datalake and whether we need to control / influence / understand the way that the FTS arranges traffic to see if it can take I/O limits into account.