pgsql: Fix theoretical torn page hazard.

Fix theoretical torn page hazard.

The original report was concerned with a possible inconsistency
between the heap and the visibility map, which I was unable to
confirm. The concern has been retracted.

However, there did seem to be a torn page hazard when using
checksums. By not setting the heap page LSN during redo, the
protections of minRecoveryPoint were bypassed. Fixed, along with a
misleading comment.

It may have been impossible to hit this problem in practice, because
it would require a page tear between the checksum and the flags, so I
am marking this as a theoretical risk. But, as discussed, it did
violate expectations about the page LSN, so it may have other
consequences.

Backpatch to all supported versions.

Reported-by: Konstantin Knizhnik
Reviewed-by: Konstantin Knizhnik
Discussion: https://postgr.es/m/fed17dac-8cb8-4f5b-d462-1bb4908c029e@garret.ru
Backpatch-through: 11

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/f893af496100737b7fa1ef861ac8bd2705b4d5f1

Modified Files
--------------
src/backend/access/heap/heapam.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)