Add multi-CPU Affinity option support for Linux (not for FreeBSD)

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  master

• Issues fixed (if any): Support affinity/pinning of parallel flows to different CPUs #1738

• Brief description of code changes (suitable for use as a commit message):

Suggested support for multi-CPU Affinity option on Linux. The PR is only for Linux, since I don't have FreeBSD machine.

The format of a CPUs list is using / as a separator ("n1/n1/...") since , is already used to separate between the client and server CPUs lists.

The fix is backward compatible, passing the CPUs list to the server in a new "server_cpu_set" JSON item and passing the first CPU defined in the server's list in the legacy "server_affinity" item.

[junousi]

[To improve SNR, question removed.]

[davidBar-On]

Hello @junousi, it is usually better to handle such discussions in iperf3 "discussion" (see "Discussions" at the top line). However, it this case I think that it is better to handle this discussion in other forums, probably a forum about NUMA (I am not familiar with this topic).

One suggestion about iperf3 is to make sure that the different 8 threads per stream are running on different CPU. E.g. this can be done by running the iperf3 test for long enough time (using the -t option) and in parallel running htop command to see the load of each CPU.