Improve and fix LATENCY query handling

This now follows the design docs everywhere, especially the maximum latency
handling.

https://bugzilla.gnome.org/show_bug.cgi?id=744106
diff --git a/gst/rtsp/gstrtpdec.c b/gst/rtsp/gstrtpdec.c
index b59aa5d..e24927b 100644
--- a/gst/rtsp/gstrtpdec.c
+++ b/gst/rtsp/gstrtpdec.c
@@ -372,6 +372,7 @@
     case GST_QUERY_LATENCY:
     {
       /* we pretend to be live with a 3 second latency */
+      /* FIXME: Do we really have infinite maximum latency? */
       gst_query_set_latency (query, TRUE, 3 * GST_SECOND, -1);
       res = TRUE;
       break;