Add optimistic locking and timestamp coverage for positioned list items

[ollym]

We use optimistic locking on tables that also do positioning, the way positioning was done (using update_all) breaks optimistic locking because it increments the lock_version meaning any already loaded models become stale.

So I've migrated to using a lower-level Arel::UpdateManager instead and made sure to also take care of timestamp columns (updated_at etc.) for those who also use caching.

Finally we no longer needed quoted_column which was a bit hacky to begin with, so I removed that method.

[ollym]

@brendon I added a fix for Rails 6.1 if you'd like to rerun tests, thanks

[ollym]

Same here, tests pass locally, unclear what's wrong with CI

$ DB=postgresql rake test
Run options: --seed 29421

# Running:

...............................................................................................................................................................................

Finished in 8.557468s, 20.4500 runs/s, 368.8007 assertions/s.

175 runs, 3156 assertions, 0 failures, 0 errors, 0 skips