Deletion of data is uncommon in our database, so the introduction of sub agents
and the deletion of them introduced issues with foreign key assumptions, as can
be seen in coder/internal#685. We could have only addressed the specific case by
allowing cascade deletion of stats as well as handling in the stats collector,
but it's unclear how many more such edge-cases we could run into.
In this change, we mark the rows as deleted via boolean instead, and filter them
out in all relevant queries.
Fixescoder/internal#685
511fd09582