sensor: minor fixes


git-svn-id: https://gnunet.org/svn/gnunet@34042 140774ce-b5e7-0310-ab8b-a85725594a96
xdht: track topology successor and predecessor neighbours.


git-svn-id: https://gnunet.org/svn/gnunet@34043 140774ce-b5e7-0310-ab8b-a85725594a96