thread-pool: Unref source after mainloop has quit to avoid races in GLib
thread-pool: Unref source after mainloop has quit to avoid races in GLib

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=723741

surround-io/gst-rtsp-server,barisdemiray/gst-rtsp-server,Lachann/gst-rtsp-server,hsteinhaus/gst-rtsp-server,thaytan/gst-rtsp-server,thaytan/gst-rtsp-server,surround-io/gst-rtsp-server,barisdemiray/gst-rtsp-server,Lachann/gst-rtsp-server,barisdemiray/gst-rtsp-server,surround-io/gst-rtsp-server,hsteinhaus/gst-rtsp-server,hsteinhaus/gst-rtsp-server,Lachann/gst-rtsp-server,thaytan/gst-rtsp-server