pgsql: Enable parallelism for prepared statements and extended query pr

Enable parallelism for prepared statements and extended query protocol.

Parallel query can't handle running a query only partially rather than
to completion. However, there seems to be no way to run a statement
prepared via SQL PREPARE other than to completion, so we can enable it
there without a problem.

The situation is more complicated for the extend query protocol.
libpq seems to provide no way to send an Execute message with a
non-zero rowcount, but some other client might. If that happens, and
a parallel plan was chosen, we'll execute the parallel plan without
using any workers, which may be somewhat inefficient but should still
work. Hopefully this won't be a problem; users can always set
max_parallel_degree=0 to avoid choosing parallel plans in the first
place.

Amit Kapila, reviewed by me.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/57a6a72b6bc98f3003e87bc31de4b9c2c89fe019

Modified Files
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src/backend/commands/prepare.c | 2 +-
src/backend/tcop/postgres.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)