Check if the largest thread pool is cluster-size + delta

When running UPerf in a cluster of 4, we see thread_pool_size_largest=39. Verify that MaxOneThreadPerSender works as expected: the largest thread pool size should be 4+delta (timer threads, retransmission): ~6.
It looks as if MPerf behaves (~6 threads), but UPerf doesn't: ~26 threads