Add option not to store all received RTTs (#115)

This prevents memory bloat from a process that wants to do very long-running ping tests.

ping.go

@@ -79,6 +79,7 @@ func New(addr string) *Pinger {
 	return &Pinger{
 		Count:      -1,
 		Interval:   time.Second,
+		RecordRtts: true,
 		Size:       timeSliceLength,
 		Timeout:    time.Second * 100000,
 		Tracker:    r.Int63n(math.MaxInt64),
@@ -122,6 +123,10 @@ type Pinger struct {
 	// Number of packets received
 	PacketsRecv int
 
+	// If true, keep a record of rtts of all received packets.
+	// Set to false to avoid memory bloat for long running pings.
+	RecordRtts bool
+
 	// rtts is all of the Rtts
 	rtts []time.Duration
 
@@ -538,7 +543,9 @@ func (p *Pinger) processPacket(recv *packet) error {
 		return fmt.Errorf("invalid ICMP echo reply; type: '%T', '%v'", pkt, pkt)
 	}
 
+	if p.RecordRtts {
 		p.rtts = append(p.rtts, outPkt.Rtt)
+	}
 	handler := p.OnRecv
 	if handler != nil {
 		handler(outPkt)