fastping.go
func (p *Pinger) Stop() {
	p.debugln("Stop(): close(p.ctx.stop)")
	close(p.ctx.stop)
	p.debugln("Stop(): <-p.ctx.done")
	<-p.ctx.done
}
func (p *Pinger) Stop() {
	p.debugln("Stop(): close(p.ctx.stop)")
	close(p.ctx.stop)
	p.debugln("Stop(): <-p.ctx.done")
	<-p.ctx.done
}
// Stop stops RunLoop(). It must be called after RunLoop(). If not, it causes
// panic.