pgsql: Add parallel_leader_participation GUC.

Add parallel_leader_participation GUC.

Sometimes, for testing, it's useful to have the leader do nothing but
read tuples from workers; and it's possible that could work out better
even in production.

Thomas Munro, reviewed by Amit Kapila and by me. A few final tweaks
by me.

Discussion: http://postgr.es/m/CAEepm=2U++Lp3bNTv2Bv_kkr5NE2pOyHhxU=G0YTa4ZhSYhHiw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e5253fdc4f5fe2f38aec47e08c6aee93f934183d

Modified Files
--------------
doc/src/sgml/config.sgml | 26 ++++++
src/backend/executor/nodeGather.c | 8 +-
src/backend/executor/nodeGatherMerge.c | 6 +-
src/backend/optimizer/path/costsize.c | 12 ++-
src/backend/optimizer/plan/planner.c | 1 +
src/backend/utils/misc/guc.c | 10 +++
src/backend/utils/misc/postgresql.conf.sample | 1 +
src/include/optimizer/planmain.h | 1 +
src/test/regress/expected/select_parallel.out | 113 ++++++++++++++++++++++++++
src/test/regress/sql/select_parallel.sql | 36 ++++++++
10 files changed, 205 insertions(+), 9 deletions(-)