Savepoints throttled our Postgres queue

Posting this on behalf of my colleague (sadly ex as of yesterday, this was his swan song) Rory and team who did a bunch of really interesting digging into why Postgres was struggling with our queues.

I'd dealt with multixact about a decade ago when I had to dig into the Postgres source to figure out how they worked, they're quite a hidden/undocumented (or at least were back then) part of the system.

Glad he spent the time writing this up.