Re: Add 64-bit XIDs into PostgreSQL 15

Re:    The "prepare" approach was the first tried.
    https://github.com/postgrespro/pg_pageprep
    But it appears to be very difficult and unreliable.  After investing
    many months into pg_pageprep, "double xmax" approach appears to be
    very fast to implement and reliable.

I'd still like a plan to retire the "double xmax" representation eventually.  Previously I suggested that this could be
doneas a post-process, before upgrade is complete, but that could potentially make upgrade very slow. 
 

Another way to retire the "double xmax" representation eventually could be to disallow "double xmax" pages in
subsequentmajor version upgrades (e.g. to PG16, if "double xmax" pages are introduced in PG15).  This gives the luxury
oftime after a fast upgrade to convert all pages to contain the epochs, while still providing a path to more
maintainablecode in the future.