Setting shutdown asyncio event in a thread-safe manner

[oandreeva-nv]

It seems like we need to be careful, when using asyncio with multi-threading. The non-graceful shutdown was happening due to "RuntimeError: Non-thread-safe operation invoked on an event loop other than the current one\n\nA" (I started asyncio.run with debug=True). Thus I'm replacing asyncio with threading.

Tests: https://gitlab-master.nvidia.com/dl/dgx/tritonserver/-/jobs/129814805
tests ran for branch oandreeva_metrics_refactor, but the change is the same. I suggest to merge this commit to the current perf PR and merge perf PR to main, i.e. not wait for metrics follow up.

[kthui]

Can you give this a quick try?

def finalize(self):
    ...
    self._event_loop.call_soon_threadsafe(self._llm_engine_shutdown_event.set)
    ...

It lets the asyncio event to be set in the same loop that the event is blocked on, so we don't need a "busy" timeout loop for checking the shutdown event.