default void onStopFollowing(final PeerId oldLeaderId, final long oldTerm) {
        // NO-OP
    }
This method is called when a follower stops following a leader and its leaderId becomes null,
situations including:
1. Handle election timeout and start preVote
2. Receive requests with higher term such as VoteRequest from a candidate
     or appendEntries request from a new leader
3. Receive timeoutNow request from current leader and start request vote.

The parameters gives the information(leaderId and term) about the very leader
whom the follower followed before.
User can reset the node's information as it stops following some leader.

@param oldLeaderId the old leader id whom the follower followed before
@param oldTerm     the old term
jraft-rheakv/rheakv-core/src/main/java/com/alipay/sofa/jraft/rhea/LeaderStateListener.java