enable packet pacing, for packets sent before handshake completion

It's not clear why this was disabled so far. The pacer should have some
allowance for bursts, and it is expected that the handshake flights fit into
this burst budget and therefore won't be delayed by the pacer.

However, when using 0-RTT, it actually makes sense to start pacing right
away, to avoid inducing packet loss very early in the connection.

connection.go

@@ -1739,7 +1739,7 @@ func (s *connection) sendPackets() error {
 	var sentPacket bool // only used in for packets sent in send mode SendAny
 	for {
 		sendMode := s.sentPacketHandler.SendMode()
-		if sendMode == ackhandler.SendAny && s.handshakeComplete && !s.sentPacketHandler.HasPacingBudget() {
+		if sendMode == ackhandler.SendAny && !s.sentPacketHandler.HasPacingBudget() {
 			deadline := s.sentPacketHandler.TimeUntilSend()
 			if deadline.IsZero() {
 				deadline = deadlineSendImmediately

connection_test.go

@@ -1771,6 +1771,7 @@ var _ = Describe("Connection", func() {
 		sph.EXPECT().GetLossDetectionTimeout().AnyTimes()
 		sph.EXPECT().SendMode().Return(ackhandler.SendAny).AnyTimes()
 		sph.EXPECT().TimeUntilSend().Return(time.Now()).AnyTimes()
+		sph.EXPECT().HasPacingBudget().Return(true).AnyTimes()
 		gomock.InOrder(
 			sph.EXPECT().SentPacket(gomock.Any()).Do(func(p *ackhandler.Packet) {
 				Expect(p.EncryptionLevel).To(Equal(protocol.EncryptionInitial))