Arvados

It seems excessive to give link objects a whole trash_at/delete_at lifecycle.

We could instead have an optional entry in properties "delete_at", this would store a UTC timestamp, and then the "trash sweep" process would look for expired links and destroy them.

With newer Postgres JSON support, I think we can write this directly as a query -- selecting the value as a JSON path, converting it to a datetime, and comparing it to the current time.

It may need to iterate over every row that has delete_at, which is probably fine unless there are many thousands of temporary permissions -- but it may now be possible now to index json path queries.