raft/raft.go
func (r *raft) promotable() bool {
	_, ok := r.prs[r.id]
	return ok
}
func (r *raft) promotable() bool {
	_, ok := r.prs[r.id]
	return ok
}
// promotable indicates whether state machine can be promoted to leader,
// which is true when its own id is in progress list.