api.go
func (r *Raft) LastContact() time.Time {
	r.lastContactLock.RLock()
	last := r.lastContact
	r.lastContactLock.RUnlock()
	return last
}
func (r *Raft) LastContact() time.Time {
	r.lastContactLock.RLock()
	last := r.lastContact
	r.lastContactLock.RUnlock()
	return last
}
// LastContact returns the time of last contact by a leader.
// This only makes sense if we are currently a follower.