Re: Add 64-bit XIDs into PostgreSQL 15

Rebase @ 53eff471c69dc8b0c01

I revised parts of heap_convert.c's conversion from 32-bit to
64-bit pages.

Previously, we used a deferred full-page write to the converted pages
to reduce the impact on WAL. Leaving some converted pages in RAM
memory. This, however, may cause the incoming autovacuum (AV) to clean
transaction statuses for some pages depending on their XIDs. As a
result, we risk losing transaction statuses and encountering problems
like "could not open file pg_xact/...".

So I decided to abandon the delayed FPW of converted pages, as this
write is purely a one-time issue on the first read of the data after
the 64-bit upgrade.