pgsql: Improve planner's estimation of the space needed for HashAgg

Log Message:
-----------
Improve planner's estimation of the space needed for HashAgg plans:
look at the actual aggregate transition datatypes and the actual overhead
needed by nodeAgg.c, instead of using pessimistic round numbers.
Per a discussion with Michael Tiemann.

Tags:
----
REL8_0_STABLE

Modified Files:
--------------
    pgsql/src/backend/executor:
        nodeAgg.c (r1.126.4.1 -> r1.126.4.2)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeAgg.c.diff?r1=1.126.4.1&r2=1.126.4.2)
    pgsql/src/backend/optimizer/plan:
        planner.c (r1.177 -> r1.177.4.1)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/planner.c.diff?r1=1.177&r2=1.177.4.1)
    pgsql/src/backend/optimizer/util:
        clauses.c (r1.186 -> r1.186.4.1)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/clauses.c.diff?r1=1.186&r2=1.186.4.1)
    pgsql/src/include/executor:
        nodeAgg.h (r1.23 -> r1.23.4.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/nodeAgg.h.diff?r1=1.23&r2=1.23.4.1)
    pgsql/src/include/optimizer:
        clauses.h (r1.77 -> r1.77.4.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/optimizer/clauses.h.diff?r1=1.77&r2=1.77.4.1)