move calculation of the PTO to the rttStats

The PTO value is needed at various places in the protocol. Calculating
it on the rttStats struct will allow us to pass around this struct.

internal/ackhandler/sent_packet_handler_test.go

@@ -611,20 +611,6 @@ var _ = Describe("SentPacketHandler", func() {
 	})
 
 	Context("probe packets", func() {
-		It("uses the RTT from RTT stats", func() {
-			rtt := 2 * time.Second
-			updateRTT(rtt)
-			Expect(handler.rttStats.SmoothedOrInitialRTT()).To(Equal(2 * time.Second))
-			Expect(handler.rttStats.MeanDeviation()).To(Equal(time.Second))
-			Expect(handler.computePTOTimeout()).To(Equal(time.Duration(2+4) * time.Second))
-		})
-
-		It("uses the granularity for short RTTs", func() {
-			rtt := time.Microsecond
-			updateRTT(rtt)
-			Expect(handler.computePTOTimeout()).To(Equal(rtt + protocol.TimerGranularity))
-		})
-
 		It("implements exponential backoff", func() {
 			sendTime := time.Now().Add(-time.Hour)
 			handler.SentPacket(ackElicitingPacket(&Packet{PacketNumber: 1, SendTime: sendTime}))