Consider implementing a version of before-update that doesn't fetch the originals

[camsaul]

Sometimes all you want to do is add a key like :updated_at (t/offset-date-time), there is really no need to fetch the original instance in this case. Seems like a significant performance win.

But really the last thing we need to do is make Toucan 2 even more complicated, so maybe we can noodle and think of a way that we can do this automagically. Can before-update determine whether any columns are accessed from the original object somehow, and only fetch it if they are? TBD.