Bug - Mixed speed broadband only measures the Maximum upload not the download

# Context Testing from WFH Workstations at 1Gbps Down and 50Mbps up to Business Network Connection at 1Gbps Up/Down

* Version of iperf3: 3.1.3

* Hardware: mixed

* Operating system (and distribution, if any): Windows

# Bug Report

* Expected Behavior - would expect that the results would show a download bandwidth of 1000Mbps and and up of 50Mbps

* Actual Behavior - Both up and down seem to be getting the max upload speed of 50Mbps, (works fine between local network devices)

* Steps to Reproduce - Run on consumer broadband with different upload and download speeds

* Possible Solution - I am guessing it is sending traffic up and pulling the same traffic down causing the bottle neck on the upload speed. maybe it would be better to initiate a download from the server and initiate a upload to the server from the client.

# Enhancement Request

* Current behavior

* Desired behavior - to clearly state the upload speed and download speed, in a normal LAN this could also indicate a wireing issue if it is not the same.

* Implementation notes