core: be more robust while scanning metadata
core: be more robust while scanning metadata

 - Add a timeout to check for stuck/aborted scannings
 - Don't reuse playbin, this seems to be problematic

GNOME/rygel,Pelagicore/Media-Manager-Rygel-patched,jku/rygel,GNOME/rygel,jku/rygel,luisbg/rygel,Tarnyko/rygel,GNOME/rygel,Pelagicore/Media-Manager-Rygel-patched,Tarnyko/rygel,Pelagicore/Media-Manager-Rygel-patched