[DeepSeek R1] Gracefully shutdown mooncake store when exception for kill

[jerrychenhf]

This is to handle a lot of shutdown cases when any processes are killed or there are exceptions.
User can use "kill pid" to kill any process and the moon cake store shutdown gracefully. WARNING: if you use kill -9 to kill a process, the process is killed forcefully and there is no way to handle. You need to restart all the services for this case. It is suggest for you to gracefully shutdown a service by "kill" the pid of the api_server. This is enough to shutdown all distributed processes started as part of the api server.
When engine process shut down, it will cleanup the following:

1. Mooncake store: Unregister the global memory
2. The Parallel environments: PyTorch distributed groups