Combine freezing and pruning steps in VACUUM

Execute both freezing and pruning of tuples in the same
heap_page_prune() function, now called heap_page_prune_and_freeze(),
and emit a single WAL record containing all changes. That reduces the
overall amount of WAL generated.

This moves the freezing logic from vacuumlazy.c to the
heap_page_prune_and_freeze() function. The main difference in the
coding is that in vacuumlazy.c, we looked at the tuples after the
pruning had already happened, but in heap_page_prune_and_freeze() we
operate on the tuples before pruning. The heap_prepare_freeze_tuple()
function is now invoked after we have determined that a tuple is not
going to be pruned away.

VACUUM no longer needs to loop through the items on the page after
pruning. heap_page_prune_and_freeze() does all the work. It now
returns the list of dead offsets, including existing LP_DEAD items, to
the caller. Similarly it's now responsible for tracking 'all_visible',
'all_frozen', and 'hastup' on the caller's behalf.

Author: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://www.postgresql.org/message-id/20240330055710.kqg6ii2cdojsxgje@liskov

src/backend/access/heap/heapam_handler.c

@@ -1122,7 +1122,7 @@ heapam_scan_analyze_next_tuple(TableScanDesc scan, TransactionId OldestXmin,
 		 * We ignore unused and redirect line pointers.  DEAD line pointers
 		 * should be counted as dead, because we need vacuum to run to get rid
 		 * of them.  Note that this rule agrees with the way that
-		 * heap_page_prune() counts things.
+		 * heap_page_prune_and_freeze() counts things.
 		 */
 		if (!ItemIdIsNormal(itemid))
 		{