pgsql: Fix parallel-safety code for parallel aggregation.

Fix parallel-safety code for parallel aggregation.

has_parallel_hazard() was ignoring the proparallel markings for
aggregates, which is no good.  Fix that.  There was no way to mark
an aggregate as actually being parallel-safe, either, so add a
PARALLEL option to CREATE AGGREGATE.

Patch by me, reviewed by David Rowley.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/41ea0c23761ca108e2f08f6e3151e3cb1f9652a1

Modified Files
--------------
doc/src/sgml/ref/create_aggregate.sgml         |  9 +++++++++
src/backend/catalog/pg_aggregate.c             |  5 +++--
src/backend/commands/aggregatecmds.c           | 21 ++++++++++++++++++++-
src/backend/commands/functioncmds.c            |  5 ++---
src/backend/optimizer/util/clauses.c           |  7 +++++++
src/include/catalog/pg_aggregate.h             |  3 ++-
src/test/regress/expected/create_aggregate.out | 12 ++++++++++--
src/test/regress/sql/create_aggregate.sql      | 12 ++++++++++--
8 files changed, 63 insertions(+), 11 deletions(-)