Add optional setting of server congestion control algorithm

[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): #1895

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

Suggested approach for adding optional parameter to -C argument that allows to set the server's TCP Congestion Control algorithm:

1. Even if the client does not support setting the algorithm (e.g. client running in Windows).
2. To a different algorithm than the client's one.

The proposed change includes error/warning checks for cases were setting the algorithm is not supported or is set for non-sending client/server.