Fix a couple of bugs with wal_log_hints.

1. Replay of the WAL record for setting a bit in the visibility map
contained an assertion that a full-page image of that record type can only
occur with checksums enabled. But it can also happen with wal_log_hints, so
remove the assertion. Unlike checksums, wal_log_hints can be changed on the
fly, so it would be complicated to figure out if it was enabled at the time
that the WAL record was generated.

2. wal_log_hints has the same effect on the locking needed to read the LSN
of a page as data checksums. BufferGetLSNAtomic() didn't get the memo.

Backpatch to 9.4, where wal_log_hints was added.

Author: hlinnaka

src/backend/access/heap/heapam.c

@@ -7479,10 +7479,11 @@ heap_xlog_visible(XLogReaderState *record)
 	{
 		/*
 		 * We don't bump the LSN of the heap page when setting the visibility
-		 * map bit (unless checksums are 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 we're not
-		 * inspecting the existing page contents in any way, we don't care.
+		 * 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
+		 * we're not inspecting the existing page contents in any way, we
+		 * don't care.
 		 *
 		 * However, all operations that clear the visibility map bit *do* bump
 		 * the LSN, and those operations will only be replayed if the XLOG LSN
@@ -7497,10 +7498,10 @@ heap_xlog_visible(XLogReaderState *record)
 	else if (action == BLK_RESTORED)
 	{
 		/*
-		 * If heap block was backed up, restore it. This can only happen with
-		 * checksums enabled.
+		 * If heap block was backed up, we already restored it and there's
+		 * nothing more to do. (This can only happen with checksums or
+		 * wal_log_hints enabled.)
 		 */
-		Assert(DataChecksumsEnabled());
 	}
 	if (BufferIsValid(buffer))
 		UnlockReleaseBuffer(buffer);

src/backend/storage/buffer/bufmgr.c

@@ -2496,7 +2496,7 @@ BufferGetLSNAtomic(Buffer buffer)
 	/*
 	 * If we don't need locking for correctness, fastpath out.
 	 */
-	if (!DataChecksumsEnabled() || BufferIsLocal(buffer))
+	if (!XLogHintBitIsNeeded() || BufferIsLocal(buffer))
 		return PageGetLSN(page);
 
 	/* Make sure we've got a real buffer, and that we hold a pin on it. */