Default executor should commit suicide if it cannot receive HTTP responses for LAUNCH_NESTED_CONTAINER calls.

If there is a network problem (e.g., a routing problem), it is possible that the agent has received LAUNCH_NESTED_CONTAINER calls from the default executor and launched the nested container, but the executor does not get the HTTP response. This would result in tasks stuck at TASK_STARTING forever. We should consider making the default executor commit suicide if it does not receive the response in a reasonable amount of time.