Index padding optimization

Attached is a patch that removes undesired paddings from b-tree indexes.

The tuples of b-tree index consist of BTItemData and index keys. BTItemData
should be placed only 2 byte alignment, so if the alignment of keys are less
than MAXIMUM_ALIGNOF, we can place them with their minimum alignment instead
of MAXALIGN.

I tested this patch with pgbench on the machine where MAXIMUM_ALIGNOF is 8.
It saved 20% of index file sizes, because accounts_pkey is btree index on
an integer (4 bytes), so the size for one tuple changed as follows:
- original : 20 bytes = ItemIdData(4) + BTItemData(8) + key(4) + padding(4)
- patched : 16 bytes = ItemIdData(4) + BTItemData(8) + key(4)

./pgbench -i -s 100
# select relpages from pg_class where relname='accounts_pkey';
- original : relpages = 27422
- patched : relpages = 21899

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ITAGAKI Takahiro
NTT Cyber Space Laboratories