pgsql: Fix problems with the "role" GUC and parallel query.

Fix problems with the "role" GUC and parallel query.

Without this fix, dropping a role can sometimes result in parallel
query failures in sessions that have used "SET ROLE" to assume the
dropped role, even if that setting isn't active any more.

Report by Pavan Deolasee. Patch by Amit Kapila, reviewed by me.

Discussion: http://postgr.es/m/CABOikdOomRcZsLsLK+Z+qENM1zxyaWnAvFh3MJZzZnnKiF+REg@mail.gmail.com

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/69125c883df30e6232412d35bf7c89b8c3ad1305

Modified Files
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src/backend/access/transam/parallel.c | 11 +++++++++++
src/backend/utils/misc/guc.c | 25 +++++++++----------------
src/include/utils/guc.h | 1 +
src/test/regress/expected/select_parallel.out | 16 ++++++++++++++++
src/test/regress/sql/select_parallel.sql | 11 +++++++++++
5 files changed, 48 insertions(+), 16 deletions(-)