fix: replace anyio.Lock with asyncio.Lock to resolve lock handling issues

[sergey21000]

Fixes #1861

Using llama_cpp.server with "stream": true parameter resulted in RuntimeError: The current task is not holding this lock error.
Using asyncio.Lock() instead of anyio.Lock() fixes this error.

[agronholm]

If this happens, then something else is being done wrong. Simply bypassing such checks isn't going to fix the root issue.