pgsql: postgres_fdw: Avoid "cursor can only scan forward" error.

postgres_fdw: Avoid "cursor can only scan forward" error.

Commit d844cd75a disallowed rewind in a non-scrollable cursor to resolve
anomalies arising from such a cursor operation.  However, this failed to
take into account the assumption in postgres_fdw that when rescanning a
foreign relation, it can rewind the cursor created for scanning the
foreign relation without specifying the SCROLL option, regardless of its
scrollability, causing this error when it tried to do such a rewind in a
non-scrollable cursor.  Fix by modifying postgres_fdw to instead
recreate the cursor, regardless of its scrollability, when rescanning
the foreign relation.  (If we had a way to check its scrollability, we
could improve this by rewinding it if it is scrollable and recreating it
if not, but we do not have it, so this commit modifies it to recreate it
in any case.)

Per bug #17889 from Eric Cyr.  Devrim Gunduz also reported this problem.
Back-patch to v15 where that commit enforced the prohibition.

Reviewed by Tom Lane.

Discussion: https://postgr.es/m/17889-e8c39a251d258dda%40postgresql.org
Discussion: https://postgr.es/m/b415ac3255f8352d1ea921cf3b7ba39e0587768a.camel%40gunduz.org

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d97f2ee50e49046a984665fb72fa14a06bd58435

Modified Files
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contrib/postgres_fdw/expected/postgres_fdw.out | 45 ++++++++++++++++++++++++++
contrib/postgres_fdw/postgres_fdw.c            | 20 +++++++++---
contrib/postgres_fdw/sql/postgres_fdw.sql      | 25 ++++++++++++++
3 files changed, 85 insertions(+), 5 deletions(-)