Provide better support for transient state machine implementation #881
Description
An application might implement a pure in-memory state machine, which will be discarded when server is shutdown, and will be rebuilt from the last snapshot the next time server starting up.
In such a scenario, the state of the state machine falls back to a previous state after restart. And openraft should help the app to rebuild the state machine to the latest state.
TODO:
- Upon startup, a node that is leader should restore local log progress and re-apply log entries if it is single node cluster #883
- Consider informing the storage to persist the
last_applied_log_id, even when applying log entry to a pure in-memory state machine; And reload the persistedlast_applied_log_idwhen starting up;