End-user friendly description of the problem this fixes or functionality that this introduces

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below
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Give a summary of what the PR does, explaining any non-trivial design decisions

The while True here is causing a lot of problems. It's easy for it to get stuck permanently.

Removing that loop is a surprisingly simple change--we just _step the agent whenever a new event comes in that it might react to. Haven't fully tested this yet but at first blush it's working well.

However, doing that surfaced a bunch of subtle issues with EventStream callbacks blocking each other, as well as main parts of the app. So now:

• All events are added to a queue, which is processed in order
• The queue is processed on its own thread
• Every subscriber gets its own thread, with an asyncio loop set up for it

Link of any specific issues this addresses

To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:ac520c1-nikolaik   --name openhands-app-ac520c1   docker.all-hands.dev/all-hands-ai/openhands:ac520c1