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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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f74f871b80a13ef72b19e3d829a109f8df0df792

Modified Files
--------------
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 | 14 ++++++++++++++
src/test/regress/sql/select_parallel.sql | 8 ++++++++
5 files changed, 43 insertions(+), 16 deletions(-)