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 The concern has been retracted.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

src/backend/access/heap/heapam.c

@@ -8823,8 +8823,7 @@ heap_xlog_visible(XLogReaderState *record)
 		/*
 		 * We don't bump the LSN of the heap page when setting the visibility
 		 * map bit (unless checksums or wal_hint_bits is enabled, in which
-		 * case we must), because that would generate an unworkable volume of
-		 * full-page writes.  This exposes us to torn page hazards, but since
+		 * case we must). This exposes us to torn page hazards, but since
 		 * we're not inspecting the existing page contents in any way, we
 		 * don't care.
 		 */
@@ -8832,6 +8831,9 @@ heap_xlog_visible(XLogReaderState *record)
 
 		PageSetAllVisible(page);
 
+		if (XLogHintBitIsNeeded())
+			PageSetLSN(page, lsn);
+
 		MarkBufferDirty(buffer);
 	}
 	else if (action == BLK_RESTORED)