Raft 节点状态

Raft 节点启动后，会在 Follower 、 Candidate 、 Leader 3 个状态间转换，直到关闭 Shutdown

// RaftState captures the state of a Raft node: Follower, Candidate, Leader,
// or Shutdown.
type RaftState uint32

const (
	// Follower is the initial state of a Raft node.
	Follower RaftState = iota

	// Candidate is one of the valid states of a Raft node.
	Candidate

	// Leader is one of the valid states of a Raft node.
	Leader

	// Shutdown is the terminal state of a Raft node.
	Shutdown