pgsql: Create memory context for HashAgg with a reasonable maxBlockSize

Create memory context for HashAgg with a reasonable maxBlockSize.

If the memory context's maxBlockSize is too big, a single block
allocation can suddenly exceed work_mem. For Hash Aggregation, this
can mean spilling to disk too early or reporting a confusing memory
usage number for EXPLAN ANALYZE.

Introduce CreateWorkExprContext(), which is like CreateExprContext(),
except that it creates the AllocSet with a maxBlockSize that is
reasonable in proportion to work_mem.

Right now, CreateWorkExprContext() is only used by Hash Aggregation,
but it may be generally useful in the future.

Discussion: https://postgr.es/m/412a3fbf306f84d8d78c4009e11791867e62b87c.camel@j-davis.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/50a38f65177ea7858bc97f71ba0757ba04c1c167

Modified Files
--------------
src/backend/executor/execUtils.c | 71 +++++++++++++++++++++++++++++++---------
src/backend/executor/nodeAgg.c   |  5 +--
src/include/executor/executor.h  |  1 +
3 files changed, 58 insertions(+), 19 deletions(-)