Reduce CPU usage when test baud rate is limited

[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): 100% CPU utilization in a limited rate test #1741

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

Suggested fix to #1741 for (significantly) reduce iperf3 send CPU usage when --baud limits the sending baud rate. The suggested change uses nanosleep() or clock_nanosleep() to sleep between sends instead of using the pacing timer. This change seem to work fine because of the current usage of threads and since the select() is used only in the main thread.

clock_nanosleep() has a cleaner implementation, so its use is preferred. However, since it seem to be newer than nanosleep(), nanosleep() is used if clock_nanosleep() is not supported. Both give similar results. If both functions are not supported, then the legacy behavior is used.

The --pacing-timer option is removed. However the related variables/functions are not removed in case they are used as part of the library.

(By the way, it seems that iperf_send_mt() can be simplified. E.g. the if (sp->sender) is probably redundant. In addition, the use of burst/multisend is also probably redundant, since the sending threads do not use select().)

[bmah888]

Thanks for the pull request! The problem that this change solves is one that's kind of been bothering me ever since we implemented multi-threading, but I didn't quite have the knowledge to fix it. Also our main use cases don't use --bitrate, so admittedly we didn't put much priority on it. Your solution is a lot fewer lines of code than I expected!

I've testing this on a few hosts we have, so far so good. Just have a couple minor changes to suggest (at this point).

[bmah888]

Thanks for the changes! I might want to tweak the help message later, but the functionality looks good.