[api] UPSERT frames instead of throwing errors

[willdurand]

When we synchronize frames that have been already sync'ed (thanks to the frame IDs), then we get an error. We should use PostgreSQL UPSERT feature and use ON CONFLICT DO NOTHING on insertion, so that we don't get an error for duplicate frames.

I guess it makes sense since the endpoint for persisting frames is called bulk insert, so it should be tolerant.