raft/node.go
func (n *node) Tick() {
	select {
	case n.tickc <- struct{}{}:
	case <-n.done:
	default:
		n.logger.Warningf("A tick missed to fire. Node blocks too long!")
	}
}
func (n *node) Tick() {
	select {
	case n.tickc <- struct{}{}:
	case <-n.done:
	default:
		n.logger.Warningf("A tick missed to fire. Node blocks too long!")
	}
}
// Tick increments the internal logical clock for this Node. Election timeouts
// and heartbeat timeouts are in units of ticks.