Investigate elevated `db_duration_s`during enabled `use_traversal_ids` FF

From &5296 (comment 547297300):

I disabled the FF after 45 minutes as it produced worrying maximum db duration stats. This graph illustrates the point:

The offending queries were related to epics...

I wasn't able to investigate the offending queries as I was pulled in to a priority incident.

It's possible the queries are spanning hierarchies (e.g. public projects) and because we have '{}' traversal_ids outside of the gitlab-org group, we could greatly increase the scope of the query.