Support affinity/pinning of parallel flows to different CPUs

# Enhancement Request

* Current behavior
iperf3 supports setting affinity to a single CPU, which made sense when it was single threaded

* Desired behavior
Support setting a rage/list of CPUs to affinity of multiple flows: Ex: "iperf3 -c localhost -P 4 -A 1-4,1-4"

* Implementation notes
There are at least two way this can be implementated and I'd suggest the second approach:
1) Replicate numactl behavior, where "numactl -C 1-4" would bind each flow to the CPU range "1-4" and Linux would be responsible for the scheduling, allowing for dinamically rebalancing the flows but generating noise and perfromance drops.
2) Statically schedule each flow to a different CPU, i.e. in a round-robin fashion, so that when you have a maching set of parallel threads and CPUs you'd have 1 flow per CPU, minimizing reschedules during the transfer and optimizing performance

Another note, is that ideally you should be able to explicitly define a CPU list, which normally uses comma as a separator, i.e. "1,3,5,7". But this would impact either in I) changing the current client/server CPU separator ("iperf3 -c localhost -P 4 -A 1,3,5,7/2,4,6,8") or to use a different delimiter character for defining the list.