Fix parallel-safety code for parallel aggregation.

Robert Haas <rhaas@postgresql.org>

Commit: 41ea0c23761ca108e2f08f6e3151e3cb1f9652a1
Author: Robert Haas <rhaas@postgresql.org>
Date: 2016-04-05T20:06:15Z
Releases: 9.6.0
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.

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