Don't hold lock unless necessary in PoolByteStream.close()

[leventov]

Summary

Don't hold lock unless necessary in PoolByteStream.close().

This lock causes significant contention and slowdown in LiteLLM when used with streaming but sync HTTP transport.

Testing: this patch was validated through load testing that decreased lock contention (validated with py-spy) and significantly decreased wall clock completion time of concurrent streaming LLM requests via LiteLLM.

Checklist

• [x] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)

[ilovelinux]

Maybe related issue:

• #1029