Prevent asynchronous execution of direct foreign-table modifications.

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: http://postgr.es/m/CAPmGK158e9sJOfuWxfn%2B0ynrspXQU3JhNjSCbaoeSzMvnga%2Bbw%40mail.gmail.com