Log receiver close. (#2456)

* Log receiver close.

This is going to increase log volume, but want to check if peer
connection close trickles back into receiver close.

* log final close

pkg/rtc/mediatrack.go

@@ -257,9 +257,11 @@ func (t *MediaTrack) AddReceiver(receiver *webrtc.RTPReceiver, track *webrtc.Tra
 			sfu.WithStreamTrackers(),
 		)
 		newWR.OnCloseHandler(func() {
+			t.params.Logger.Infow("webrtc receiver closed")
 			t.MediaTrackReceiver.SetClosing()
 			t.MediaTrackReceiver.ClearReceiver(mime, false)
 			if t.MediaTrackReceiver.TryClose() {
+				t.params.Logger.Infow("mediaTrack closed")
 				if t.dynacastManager != nil {
 					t.dynacastManager.Close()
 				}