func (r *Raft) configurationChangeChIfStable() chan *configurationChangeFuture {
	// Have to wait until:
	// 1. The latest configuration is committed, and
	// 2. This leader has committed some entry (the noop) in this term
	//    https://groups.google.com/forum/#!msg/raft-dev/t4xj6dJTP6E/d2D9LrWRza8J
	if r.configurations.latestIndex == r.configurations.committedIndex &&
		r.getCommitIndex() >= r.leaderState.commitment.startIndex {
		return r.configurationChangeCh
	}
	return nil
}
func (r *Raft) configurationChangeChIfStable() chan *configurationChangeFuture {
	// Have to wait until:
	// 1. The latest configuration is committed, and
	// 2. This leader has committed some entry (the noop) in this term
	//    https://groups.google.com/forum/#!msg/raft-dev/t4xj6dJTP6E/d2D9LrWRza8J
	if r.configurations.latestIndex == r.configurations.committedIndex &&
		r.getCommitIndex() >= r.leaderState.commitment.startIndex {
		return r.configurationChangeCh
	}
	return nil
}
// configurationChangeChIfStable returns r.configurationChangeCh if it's safe
// to process requests from it, or nil otherwise. This must only be called
// from the main thread.
//
// Note that if the conditions here were to change outside of leaderLoop to take
// this from nil to non-nil, we would need leaderLoop to be kicked.