pgsql: Include chunk overhead in hash table entry size estimate.

Include chunk overhead in hash table entry size estimate.

Don't try to be precise about it, just use a constant 16 bytes of
chunk overhead. Being smarter would require knowing the memory context
where the chunk will be allocated, which is not known by all callers.

Discussion: https://postgr.es/m/20200325220936.il3ni2fj2j2b45y5@alap3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/0588ee63aa2d8c5765d086991555cd9acdd4d86f

Modified Files
--------------
src/backend/executor/nodeAgg.c | 36 ++++++++++++++++++++++++++++++------
src/include/executor/nodeAgg.h |  2 +-
2 files changed, 31 insertions(+), 7 deletions(-)