Unique key for asset_value

Currently defined as:

```
-- auto-generated definition
create unique index asset_value_asset_id_evaluated_at_granularity_key
    on asset_value (asset_id, evaluated_at, granularity);
```

We're going to need `source` as the fourth element in the unique key as:

```
(asset_id, evaluated_at, granularity, source)
```