Fix infinite growth of the syncroton_relations_state table on Ping
The scenario was:
- We had a synckey with an old timestamp because the folder hasn't been synced for a long time.
- There was no relations state entry with a lower timestamp
- The folder was still included in ping
- Each ping thus did not find an entry, but then inserted a new one (with a more recent timestamp, so it was never found)
As mentioned before, we should really attach the state to the synckey
and not a timestamp, but in the meanwhile this will prevent the growth
and cleanup existing tables.