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.

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. */