Implement nuraft system_exit #852
Description
When nuraft identifies an unrecoverable critical case, it will call our system_exit callback. Currently, we only print a log which is meaningless and lead to the system stuck all the time.
Here is a concrete case:
When a follower recovers from the network issue, it attempts to request pre-vote, but it is rejected by the other peers. After rejected by quorum > 20 times, it tries to reconnect to leader. However it doesn't know who is leader and goes into system_exit.
So if we can restart the raft server which will reset all the state, it will becomes self-recoverable.
Whether we can safely restart the raft server without affecting other raft group needs more investigation and tests.