Re: optimizing pg_upgrade's once-in-each-database steps

I finished parallelizing everything in pg_upgrade I could easily
parallelize with the proposed async task API. There are a few remaining
places where I could not easily convert to the new API for whatever reason.
AFAICT those remaining places are either not showing up prominently in my
tests, or they only affect versions that are unsupported or will soon be
unsupported when v18 is released. Similarly, I noticed many of the checks
follow a very similar coding pattern, but most (if not all) only affect
older versions, so I'm unsure whether it's worth the trouble to try
consolidating them into one scan through all the databases.

The code is still very rough and nowhere near committable, but this at
least gets the patch set into the editing phase.

--
nathan