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.