pgsql: postgres_fdw: Fix handling of abort-cleanup-failed connections.

postgres_fdw: Fix handling of abort-cleanup-failed connections.

As connections that failed abort cleanup can't safely be further used,
if a remote query tries to get such a connection, we reject it.
Previously, this rejection involved dropping the connection if it was
open, without accounting for the possibility of open cursors using it,
causing a server crash when such an open cursor tried to use an
already-dropped connection, as a cursor-handling function
(create_cursor, fetch_more_data, or close_cursor) was called on a freed
PGconn. To fix, delay dropping failed connections until abort cleanup
of the main transaction, to ensure open cursors using such a connection
can safely refer to the PGconn for it.

Oversight in commit 8bf58c0d9.

Reported-by: Zhibai Song <songzhibai1234(at)gmail(dot)com>
Diagnosed-by: Zhibai Song <songzhibai1234(at)gmail(dot)com>
Author: Etsuro Fujita <etsuro(dot)fujita(at)gmail(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Matheus Alcantara <matheusssilv97(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAPmGK176y6JP017-Cn%2BhS9CEJx_6iVhRoYbAqzuLU4d8-XPPNg%40mail.gmail.com
Backpatch-through: 14

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/648818ba3876fb3e43faffea729eccba969eefab

Modified Files
--------------
contrib/postgres_fdw/connection.c | 10 ++--
contrib/postgres_fdw/expected/postgres_fdw.out | 76 ++++++++++++++++++++++++++
contrib/postgres_fdw/sql/postgres_fdw.sql | 51 +++++++++++++++++
3 files changed, 133 insertions(+), 4 deletions(-)