Various query builder fixes

[luixo]

As we recently migrated to Kysely, the query builder, we've got some issues left with it:

• a bunch of nullable values that always exist. We should either modify queries, comment why they always exist despite of type errors or make a PR to Kysely repo: #1, #2, #3, #4
• it seems like using query write pool manually (instead of through Kyselys helpers) helps with queries speed. Using helpers sometimes we had to wait up to 2 minutes for a query to complete and timing stays pretty low with a manual write pool. Sometimes the problem rises up again, so we need a thorough research on what's happening and how we can keep timings in a decent range
• some complicated values are not supported by Kysely, so we have to write them either manually or via self-built helpers and types are not reflecting actual result there. We should either write proper helpers or make a PR to Kyselys repo: distinct on, count (we may ask repo maintainer to add an overwritable module-scoped type for it's result), div