Behavior when dqlite's raft node enters the `RAFT_UNAVAILABLE` state.

[MathieuBordere]

When an unrecoverable error occurs, a raft node can enter the RAFT_UNAVAILABLE state and will never leave it, unless eventually the process running the raft node is restarted. From my understanding the app package does not yet handle this case. Ideally we should detect this unrecoverable state, and restart it.