This PR adopt the changes in https://github.com/tikv/raft-rs/pull/561 that directly reset the apply_unpersisted_log_limit limit in raft when leader is demoted. It fix the bug that in the previous impl, the reset is done in "on_role_change" which is called in handle_raft_ready, in the round that leader is demoted, it still return unpersisted comitted entries as at the time, the limit is not reset.
NOTE: once we want to support this feature on follower, we need to remove the original assert as it is incorrect in this case.

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