Multiple Users at once

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# Context
Current iPerf sever seems to only allow 1 user at a time, in some cases we are okay with some performance hit on the service if we can allow multiple users. Could we add a configure flag / build flag / etc. or command line config to set max number of users who can connect at once?

* Version of iperf3:
3.10

* Hardware:
Docker Containerization

* Operating system (and distribution, if any):
Ubuntu Docker Image 22.04

_Please note: iperf3 is supported on Linux, FreeBSD, and macOS.
Support may be provided on a best-effort basis to other UNIX-like
platforms. We cannot provide support for building and/or running
iperf3 on Windows, iOS, or Android._

* Other relevant information (for example, non-default compilers,
libraries, cross-compiling, etc.):
Compiled on demand as part of build process from source

_Please fill out one of the "Bug Report" or "Enhancement Request"
sections, as appropriate. Note that submissions of bug fixes, new
features, etc. should be done as a pull request at
https://github.com/esnet/iperf/pulls_

# Bug Report

* Expected Behavior

* Actual Behavior

* Steps to Reproduce

* Possible Solution

# Enhancement Request

* Current behavior
One user can access server at a time.

* Desired behavior
Similar to iperf2, multiple users can connect at once. Even if 1 per user account, would be nice to allow multiple users to use the service at one time.

* Implementation notes
Our team is setting up a webui / etc. for this as part of https://github.com/PenguinCloud/PenguinPerf3 to be used inside our infrastructure and others and will keep this open source.