func (s *EtcdServer) sync(timeout time.Duration) {
	req := pb.Request{
		Method: "SYNC",
		ID:     s.reqIDGen.Next(),
		Time:   time.Now().UnixNano(),
	}
	data := pbutil.MustMarshal(&req)
	// There is no promise that node has leader when do SYNC request,
	// so it uses goroutine to propose.
	ctx, cancel := context.WithTimeout(s.ctx, timeout)
	s.goAttach(func() {
		s.r.Propose(ctx, data)
		cancel()
	})
}
func (s *EtcdServer) sync(timeout time.Duration) {
	req := pb.Request{
		Method: "SYNC",
		ID:     s.reqIDGen.Next(),
		Time:   time.Now().UnixNano(),
	}
	data := pbutil.MustMarshal(&req)
	// There is no promise that node has leader when do SYNC request,
	// so it uses goroutine to propose.
	ctx, cancel := context.WithTimeout(s.ctx, timeout)
	s.goAttach(func() {
		s.r.Propose(ctx, data)
		cancel()
	})
}
// sync proposes a SYNC request and is non-blocking.
// This makes no guarantee that the request will be proposed or performed.
// The request will be canceled after the given timeout.