refactor HTTP/3 stream handling to use a dedicated stream

Reading from and writing onto this stream applies HTTP/3 DATA framing.

http3/server_test.go

@@ -135,11 +135,8 @@ var _ = Describe("Server", func() {
 			buf := &bytes.Buffer{}
 			str := mockquic.NewMockStream(mockCtrl)
 			str.EXPECT().Write(gomock.Any()).DoAndReturn(buf.Write).AnyTimes()
-			closed := make(chan struct{})
-			str.EXPECT().Close().Do(func() { close(closed) })
 			rw := newRequestWriter(utils.DefaultLogger)
-			Expect(rw.WriteRequest(str, req, false, false)).To(Succeed())
-			Eventually(closed).Should(BeClosed())
+			Expect(rw.WriteRequestHeader(str, req, false)).To(Succeed())
 			return buf.Bytes()
 		}