Add concurrency option for benchmark

[cermeng]

Motivation

There could be a load balancer above the server to control the request traffic since the server can't reject requests. This pr can simulate this situation. Code borrowed from vllm-project/vllm#9390

Modifications

• add an option --max-concurrency to bench_serving.py
• make sure there will not exceed max-concurrency requests coming to the server concurrently

Checklist

• Format your code according to the Contributor Guide.
• Add unit tests as outlined in the Contributor Guide.
• Update documentation as needed, including docstrings or example tutorials.

[zhyncs]

I'll fix the SimpleNamespace issue cc @merrymercy @cermeng