etcdserver/raft.go
func (r *raftNode) tick() {
	r.tickMu.Lock()
	r.Tick()
	r.tickMu.Unlock()
}
func (r *raftNode) tick() {
	r.tickMu.Lock()
	r.Tick()
	r.tickMu.Unlock()
}
// raft.Node does not have locks in Raft package