Re: shared-memory based stats collector - v70

Hi,

Here comes v70:
- extended / polished the architecture comment based on feedback from Melanie
and David
- other polishing as suggested by David
- addressed the open issue around pgstat_report_stat(), as described in
https://www.postgresql.org/message-id/20220405204019.6yj7ocmpw352c2u5%40alap3.anarazel.de
- while working on the above point, I noticed that hash_bytes() showed up
noticeably in profiles, so I replaced it with a fixed-width function
- found a few potential regression test instabilities by either *always*
flushing in pgstat_report_stat(), or only flushing when force = true.
- random minor improvements
- reordered commits some

I still haven't renamed pg_stat_exists_stat() yet - I'm leaning towards
pg_stat_have_stats() or pg_stat_exists() right now. But it's an SQL function
for testing, so it doesn't really matter.

I think this is basically ready, minus a a few comment adjustments here and
there. Unless somebody protests I'm planning to start pushing things tomorrow
morning.

It'll be a few hours to get to the main commit - but except for 0001 it
doesn't make sense to push without intending to push later changes too. I
might squash a few commits togther.

There's lots that can be done once all this is in place, both simplifying
pre-existing code and easy new features, but that's for a later release.

Greetings,

Andres Freund