Benchmark UNICAST vs UNICAST2 vs NAKACK2 with JGroups 3.0.x

Recent changes seem to have brought performance a bit down, so we need to check whether this is caused by reverting to UNICAST instead of UNICAST2 (ISPN-1892).
Performance has decreased most in the 2-node scenario, where we were actually sending multicasts instead of unicasts. So we need to check whether there is a performance difference between UNICAST2 and NAKACK2 as well.

If we find UNICAST2 to perform better, we can proceed with ISPN-1879 on the 5.1.x branch as well as on the master branch.