Fix insert of duplicate key into event_json

[JanZerebecki]

When an incoming event id is present in event_json but not in events
synapse fails trying to insert it with "psycopg2.errors.UniqueViolation:
duplicate key value violates unique constraints", because it is only
filtered based on those that are in events.

I don't know why those become out of sync, but this happening was
reported by others before.

Fix this by using an upsert (which inserts or updates existing records)
instead of a normal insert.

Please verify that this is the safe and correct thing to do before
merging this. Verify e.g. that it doesn't allow breaking history
integrity or something like it. As I don't know enough to understand
what this change entails.

Fixes: #10718
Signed-off-by: Jan Zerebecki [email protected]

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[richvdh]

per the discussion on #10718, we're not (yet) convince that this is a change that we should be making.

[richvdh]

I think we agreed this was not a change that made sense. If your database is corrupt, then hacking around it isn't going to improve things.