D8: Fix handling of database schema updates in update manager workflow

We tried to fix this in D7 at #606190: Fix handling of database schema updates in update manager workflow → but ran out of time.

Sadly, the original issue summary there is still basically right for the problem we'd need to fix here. However, I'm opening a new issue since that one is D7-specific, done, and mostly discusses our limited options at the time. In theory, we have more time and options now, so I figured I'd start with a clean slate so we can design and implement something "delightful".

Basically, once you update existing code on your site, there might be DB updates and you might need to run update.php before your site works again. Ideally, update manager would detect this for you, and ideally just do it for you. However, this is complicated for various reasons discussed at length in #606190, in particular, see what I wrote at comments #8 → and #32 → .

So, I don't have high hopes that we're going to do much better in D8 than we have in D7. But, at least if we have an issue open about it there's a non-zero chance for improvement. ;)