public CandidateState candidateStateOrThrow() {
        if (isCandidate())
            return (CandidateState) state;
        throw new IllegalStateException("Expected to be Candidate, but current state is " + state);
    }
Become a follower of an elected leader so that we can begin fetching.
raft/src/main/java/org/apache/kafka/raft/QuorumState.java