Remove the usage of pacing_timer and simplify iperf_mt_send

[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): discussion Variable changes by main and work threads without lock protection #1736

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

Removed the usage of pacing_timer, and keep it as a parameter for backward compatibility for servers running older versions of iperf3.

In addition, simplified iperf_send_mt(), since it contained some leftovers of the multi-streams support (including some variables renaming).

Tested with and without nanosleep support.

[bmah888]

Thanks for the PR! I like the idea of this change (haven't had a chance to really study it though).

[bmah888]

Tested this on all our supported platforms, pacing (still) works for both forward and reverse direction tests. I will merge this shortly. Thanks for making iperf3 smaller and (slightly) less complicated!