Improve the gateway stability for long request

[Jeffwan]

🐛 Describe the bug

I am not sure that's all related to timeout setting. https://github.com/vllm-project/aibrix/pull/879

Steps to Reproduce

send > 20k prompts to the server with 8 * A100, QPS > 1.5 will easily trigger the issue

Expected behavior

gateway should not have any connection issues

Environment

v0.2.1