Make `last_applied_state()` into sync

[drmingdrmer]

Make last_applied_state() into sync, provided all state machines will use atomic read or the like.

sync method is fast to run. And most of the time it is not hard for an application to maintain an in-memory variable to serve this method.

https://github.com/datafuselabs/openraft/blob/46644c8409c7dff627c991cc65849507ad5265b3/openraft/src/storage.rs#L303-L308

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