use default RTT (100ms) for 0-RTT if no prior estimate (#5388)

* utils: initialize the {Smoothed, Latest, Min}RTT to 100ms

* utils: use time.Duration.Nanoseconds instead of uint64

No functional change expected.

* congestion: better check to avoid division by zero

internal/congestion/cubic_sender.go

@@ -278,8 +278,8 @@ func (c *cubicSender) isCwndLimited(bytesInFlight protocol.ByteCount) bool {
 func (c *cubicSender) BandwidthEstimate() Bandwidth {
 	srtt := c.rttStats.SmoothedRTT()
 	if srtt == 0 {
-		// If we haven't measured an rtt, the bandwidth estimate is unknown.
-		return infBandwidth
+		// This should never happen, but if it does, avoid division by zero.
+		srtt = protocol.TimerGranularity
 	}
 	return BandwidthFromDelta(c.GetCongestionWindow(), srtt)
 }

internal/congestion/cubic_sender_test.go

@@ -162,7 +162,6 @@ func TestCubicSenderExponentialSlowStart(t *testing.T) {
 	// At startup make sure we can send.
 	require.True(t, sender.sender.CanSend(0))
 	require.Zero(t, sender.sender.TimeUntilSend(0))
-	require.Equal(t, infBandwidth, sender.sender.BandwidthEstimate())
 
 	const numberOfAcks = 20
 	for range numberOfAcks {