pgsql: Prevent asynchronous execution of direct foreign-table modificat

Prevent asynchronous execution of direct foreign-table modifications.

Commits 27e1f1456 and 86dc90056, which were independently discussed,
cause a crash when executing an inherited foreign UPDATE/DELETE query
with asynchronous execution enabled, where children of an Append node
that is the direct/indirect child of the ModifyTable node are rewritten
so as to modify foreign tables directly by postgresPlanDirectModify();
as in that case the direct modifications are executed asynchronously,
which is not currently supported by asynchronous execution. Fix by
disabling asynchronous execution of the direct modifications in that
function.

Author: Etsuro Fujita
Reviewed-by: Amit Langote
Discussion: https://postgr.es/m/CAPmGK158e9sJOfuWxfn%2B0ynrspXQU3JhNjSCbaoeSzMvnga%2Bbw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a784859f4480ceaa05a00ca35311071ca33483d1

Modified Files
--------------
contrib/postgres_fdw/expected/postgres_fdw.out | 55 ++++++++++++++++++++++++++
contrib/postgres_fdw/postgres_fdw.c | 7 ++++
contrib/postgres_fdw/sql/postgres_fdw.sql | 8 ++++
3 files changed, 70 insertions(+)