Answer dead-letters on the lent serving connection, and keep one clip helper

Two handles to the same serving connection, and two copies of one helper,
would drift (review of hq issues 327 and 330).

cmd/mesh-controller/deadletters_test.go

@@ -26,9 +26,9 @@ func servingDeadLetters(t *testing.T) *broker.JetStream {
 	if err := js.EnsureControllerBuckets(); err != nil {
 		t.Fatal(err)
 	}
-	before := deadLettersOn.Load()
-	deadLettersOn.Store(&busHandles{conn: js.Conn(), js: js.Context()})
-	t.Cleanup(func() { deadLettersOn.Store(before) })
+	before := servingBus.Load()
+	servingBus.Store(js)
+	t.Cleanup(func() { servingBus.Store(before) })
 	return js
 }