This patch changes int2_avg_accum() and int4_avg_accum() use the nodeAgg

This patch changes int2_avg_accum() and int4_avg_accum() use the nodeAgg
performance hack Tom introduced recently. This means we can avoid
copying the transition array for each input tuple if these functions
are invoked as aggregate transition functions.

To test the performance improvement, I created a 1 million row table
with a single int4 column. Without the patch, SELECT avg(col) FROM
table took about 4.2 seconds (after the data was cached); with the
patch, it took about 3.2 seconds. Naturally, the performance
improvement for a less trivial query (or a table with wider rows)
would be relatively smaller.