TUN-6856: Refactor to lay foundation for tracing ICMP

Remove send and return methods from Funnel interface. Users of Funnel can provide their own send and return methods without wrapper to comply with the interface.
Move packet router to ingress package to avoid circular dependency

ingress/icmp_posix_test.go

@@ -52,24 +52,26 @@ func TestFunnelIdleTimeout(t *testing.T) {
 			},
 		},
 	}
-	responder := echoFlowResponder{
-		decoder:  packet.NewICMPDecoder(),
-		respChan: make(chan []byte),
+	muxer := newMockMuxer(0)
+	responder := packetResponder{
+		datagramMuxer: muxer,
 	}
-	require.NoError(t, proxy.Request(&pk, &responder))
-	responder.validate(t, &pk)
+	require.NoError(t, proxy.Request(ctx, &pk, &responder))
+	validateEchoFlow(t, muxer, &pk)
 
 	// Send second request, should reuse the funnel
-	require.NoError(t, proxy.Request(&pk, nil))
-	responder.validate(t, &pk)
+	require.NoError(t, proxy.Request(ctx, &pk, &packetResponder{
+		datagramMuxer: nil,
+	}))
+	validateEchoFlow(t, muxer, &pk)
 
 	time.Sleep(idleTimeout * 2)
-	newResponder := echoFlowResponder{
-		decoder:  packet.NewICMPDecoder(),
-		respChan: make(chan []byte),
+	newMuxer := newMockMuxer(0)
+	newResponder := packetResponder{
+		datagramMuxer: newMuxer,
 	}
-	require.NoError(t, proxy.Request(&pk, &newResponder))
-	newResponder.validate(t, &pk)
+	require.NoError(t, proxy.Request(ctx, &pk, &newResponder))
+	validateEchoFlow(t, newMuxer, &pk)
 
 	cancel()
 	<-proxyDone