Avg performance for int8/numeric

Avg performance for these two datatypes can be improved by *not*
calculating the sum of squares in the shared accumulator
(do_numeric_accum). However there is a little subtlety as this function
is also the shared by variance and stddev!

This patch:

- Modifies do_numeric_accum to have an extra bool parameter and does not
calc sumX2 when it is false.
- Amends all the accumulators that call it to include the bool (set to
true).
- Adds new functions [int8|numeric]_avg_accum that call do_numeric_accum
with the bool set to false.
- Amends the the bootstrap entries for pg_aggregate to use the new
accumulators for avg(int8|numeric).
- Adds the new accumulators into the bootstrap entries for pg_proc.

Performance gain is approx 33% (it is still slower than doing sum/count
- possibly due to the construct/deconstruct overhead of the numeric
transition array).

Cheers

Mark