application_settings can contain multiple rows

Today we ran into the problem on GitLab.com where application_settings contained 55 rows. On database level there is nothing preventing this from happening (e.g. a UNIQUE index). The impact thus far (as we can find) meant InfluxDB was not getting data because the 54 bogus ones were sending data to localhost.

I solved this by removing the bogus database rows, but we need to prevent this from happening on DB level.