don't return a stateless reset token when adding a new connection ID

Both server and client didn't make use of the token. It was only used by
the connIDGenerator.

session_test.go

@@ -1318,6 +1318,7 @@ var _ = Describe("Session", func() {
 			packer.EXPECT().HandleTransportParameters(params)
 			packer.EXPECT().PackPacket().MaxTimes(3)
 			Expect(sess.earlySessionReady()).ToNot(BeClosed())
+			sessionRunner.EXPECT().GetStatelessResetToken(gomock.Any()).Times(2)
 			sessionRunner.EXPECT().Add(gomock.Any(), sess).Times(2)
 			sess.processTransportParameters(params)
 			Expect(sess.earlySessionReady()).To(BeClosed())