[Zeusd] Better failure handling and testing

[jaywonchung]

Right now, zeusd assumes NVML operations will mostly succeed. However, for this to be more robust, we want to handle more failure cases. NVML might hang for some unknown reason, and we don't want the management task in zeusd (and thus a blocking request) to also hang forever. Or GPU might go lost, which will raise a specific error from NVML.

We want some timeout, a cancellation mechanism, and a way to mark the GPU as dead so that subsequent requests don't wait the full timeout. The failure will be reported, but we don't want zeusd threads to panic and burn and die.

[jaywonchung]

Timeouts for sync code cannot be done with tokio::time::timeout since it only checks deadline misses on yield points. Instead, perhaps it's a better idea to have a dedicated thread for each GPU manager (instead of the current Tokio task implementation) and use tokio::time::timeout on channel.recv, assuming channel.recv has multiple yield points internally while awaiting. Tokio channels can do sync-async communication (e.g., Receiver.blocking_recv.