Use the correct database entry instead of the frontendDB entry

crash if SLAPI plugin doesn't register a description

more cleanup; add the frontend to the set of monitored databases; handle exceptional cases a bit more smoothly

- in case we appended an entry to a yet unknown parent, we don't know
  the full DN, so unregistering needs to be possible based on the
  available data;
- complete unregister calls so that not-yet-registered stuff can be
  removed from limbo