pgsql: Fix mistaken failure to allow parallelism in corner case.

Fix mistaken failure to allow parallelism in corner case.

If we try to run a parallel plan in serial mode because, for example,
it's going to be scanned via a cursor, but for some reason we're
already in parallel mode (for example because an outer query is
running in parallel), we'd incorrectly try to launch workers.
Fix by adding a flag to the EState, so that we can be certain that
ExecutePlan() and ExecGather()/ExecGatherMerge() will have the same
idea about whether we are executing serially or in parallel.

Report and fix by Amit Kapila with help from Kuntal Ghosh. A few
tweaks by me.

Discussion: http://postgr.es/m/CAA4eK1+_BuZrmVCeua5Eqnm4Co9DAXdM5HPAOE2J19ePbR912Q@mail.gmail.com

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/965a16fa9f8c316c30f4ada3c1c98b5dd4666f25

Modified Files
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src/backend/executor/execMain.c | 1 +
src/backend/executor/execUtils.c | 2 ++
src/backend/executor/nodeGather.c | 2 +-
src/backend/executor/nodeGatherMerge.c | 2 +-
src/include/nodes/execnodes.h | 2 ++
5 files changed, 7 insertions(+), 2 deletions(-)