Here is a small patch extracted from the undo log patch set that I'd
like to discuss separately and commit soon. I'm pretty sure that
zheap, zedstore and anyone else developing new AMs based on 64 bit
xids needs this, but there are no plans to extend WAL records to
actually carry 64 bit xids yet, and I want to discourage people from
making generic xid expanding functions that don't have any
interlocking, as I mentioned recently.
It's defined in xlogreader.c, because that's where such things
naturally live, but it can only work during replay for now, so I
wrapped it in a FRONTEND invisibility cloak. That means that
front-end tools (pg_waldump.c) can't use it and will have to continue
show 32 bit xids for now.