pgsql: Allow parallel query for prepared statements with generic plans.

Allow parallel query for prepared statements with generic plans.

This was always intended to work, but due to an oversight in
max_parallel_hazard_walker, it didn't. In testing, we missed the
fact that it was only working for custom plans, where the parameter
value has been substituted for the parameter itself early enough
that everything worked. In a generic plan, the Param node survives
and must be treated as parallel-safe. SerializeParamList provides
for the transmission of parameter values to workers.

Amit Kapila with help from Kuntal Ghosh. Some changes 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/a87c0c7631d2ec738e78b909f5dcea16ea1e832f

Modified Files
--------------
src/backend/optimizer/util/clauses.c | 8 +++-
src/pl/plpgsql/src/pl_exec.c | 63 +++++++++++++++++++--------
src/test/regress/expected/select_parallel.out | 20 +++++++++
src/test/regress/sql/select_parallel.sql | 6 +++
4 files changed, 76 insertions(+), 21 deletions(-)