Save a few bytes per CatCTup

Hi,

The current code in CatalogCacheCreateEntry():

```
		ct = (CatCTup *) palloc(sizeof(CatCTup) +
								MAXIMUM_ALIGNOF + dtp->t_len);
		ct->tuple.t_len = dtp->t_len;
		ct->tuple.t_self = dtp->t_self;
		ct->tuple.t_tableOid = dtp->t_tableOid;
		ct->tuple.t_data = (HeapTupleHeader)
			MAXALIGN(((char *) ct) + sizeof(CatCTup));
		/* copy tuple contents */
		memcpy((char *) ct->tuple.t_data,
			   (const char *) dtp->t_data,
			   dtp->t_len);
```

If I understand correctly, we just want "ct->tuple.t_data" align
to MAXIMUM_ALIGNOF here. So we can save MAXIMUM_ALIGNOF
bytes per CatCTup by this:

```
		ct = (CatCTup *) palloc(MAXALIGN(sizeof(CatCTup)) +
								dtp->t_len);
		ct->tuple.t_len = dtp->t_len;
		ct->tuple.t_self = dtp->t_self;
		ct->tuple.t_tableOid = dtp->t_tableOid;
		ct->tuple.t_data = (HeapTupleHeader)
			(((char *) ct) + MAXALIGN(sizeof(CatCTup)));
		/* copy tuple contents */
		memcpy((char *) ct->tuple.t_data,
			   (const char *) dtp->t_data,
			   dtp->t_len);
```

It's correct because palloc() always return a max-aligned pointer.

--
Regards,
ChangAo Chen