pgsql: Save calculated transitionSpace in Agg node.

Save calculated transitionSpace in Agg node.

This will be useful in the upcoming Hash Aggregation work to improve
estimates for hash table sizing.

Discussion: https://postgr.es/m/37091115219dd522fd9ed67333ee8ed1b7e09443.camel%40j-davis.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c11cb17dc55a6b432dc637491a4491920f9c2770

Modified Files
--------------
src/backend/optimizer/plan/createplan.c | 9 +++++++--
src/backend/optimizer/util/pathnode.c   | 2 ++
src/include/nodes/pathnodes.h           | 2 ++
src/include/nodes/plannodes.h           | 1 +
src/include/optimizer/planmain.h        | 4 ++--
5 files changed, 14 insertions(+), 4 deletions(-)