iperf3: updated to 3.10

iperf 3.10 2021-05-26
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* Notable user-visible changes

  * Fix a bug where some --reverse tests didn't terminate.
  * Responsiveness of control connections is slightly improved.
  * The allowable clock skew when doing authentication between client
    and server is now configurable with the new --time-skew-threshold.
  * Bitrate throttling using the -b option now works when a burst size
    is specified.
  * A bug with calculating CPU utilization has been fixed.
  * A --bind-dev option to support binding sockets to a given network
    interface has been added to make iperf3 work better with
    multi-homed machines and/or VRFs.
  * --pidfile now works with --client mode.
  * The server is now less likely to get stuck due to network errors
    controlled by the new --rcv-timeout option.
  * Fixed a few bugs in termination conditions for byte or
    block-limited tests.
  * Added tcp_info.snd_wnd to JSON output.
  * Some bugs with garbled JSON output have been fixed.
  * Support for setting the IPv4 don't-fragment (DF) bit has been
    added with the new --dont-fragment option.
  * A failure with not being able to read the congestion control
    algorithm under WSL1 has been fixed.
  * Error handling and error messages now make more sense in cases
    where sockets were not successfully opened.
  * Some buffer overflow hazards were fixed.

* Notable developer-visible changes
  * It is now possible to use the API to set/get the congestion
    control algorithm.

(adam)