pgsql: Disable asynchronous execution if using gating Result nodes.

Disable asynchronous execution if using gating Result nodes.

mark_async_capable_plan(), which is called from create_append_plan() to
determine whether subplans are async-capable, failed to take into
account that the given subplan created from a given subpath might
include a gating Result node if the subpath is a SubqueryScanPath or
ForeignPath, causing a segmentation fault there when the subplan created
from a SubqueryScanPath includes the Result node, or causing
ExecAsyncRequest() to throw an error about an unrecognized node type
when the subplan created from a ForeignPath includes the Result node,
because in the latter case the Result node was unintentionally
considered as async-capable, but we don't currently support executing
Result nodes asynchronously.  Fix by modifying mark_async_capable_plan()
to disable asynchronous execution in such cases.  Also, adjust code in
the ProjectionPath case in mark_async_capable_plan(), for consistency
with other cases, and adjust/improve comments there.

is_async_capable_path() added in commit 27e1f1456, which was rewritten
to mark_async_capable_plan() in a later commit, has the same issue,
causing the error at execution mentioned above, so back-patch to v14
where the aforesaid commit went in.

Per report from Justin Pryzby.

Etsuro Fujita, reviewed by Zhihong Yu and Justin Pryzby.

Discussion: https://postgr.es/m/20220408124338.GK24419%40telsasoft.com

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ebb79024152348227250404fd06c35c56cbc672e

Modified Files
--------------
contrib/postgres_fdw/expected/postgres_fdw.out | 26 ++++++++++++++++++++++++++
contrib/postgres_fdw/sql/postgres_fdw.sql      |  5 +++++
src/backend/optimizer/plan/createplan.c        | 19 +++++++++++++------
3 files changed, 44 insertions(+), 6 deletions(-)