func (p *Pinger) RemoveIPAddr(ip *net.IPAddr) {
	p.mu.Lock()
	delete(p.addrs, ip.String())
	p.mu.Unlock()
}
func (p *Pinger) RemoveIPAddr(ip *net.IPAddr) {
	p.mu.Lock()
	delete(p.addrs, ip.String())
	p.mu.Unlock()
}
// RemoveIPAddr removes an IP address from Pinger. ip arg should be a net.IPAddr
// pointer.