pgsql: Prohibit shutting down resources if there is a possibility ofba

Prohibit shutting down resources if there is a possibility of back up.

Currently, we release the asynchronous resources as soon as it is evident
that no more rows will be needed e.g. when a Limit is filled.  This can be
problematic especially for custom and foreign scans where we can scan
backward.  Fix that by disallowing the shutting down of resources in such
cases.

Reported-by: Robert Haas
Analysed-by: Robert Haas and Amit Kapila
Author: Amit Kapila
Reviewed-by: Robert Haas
Backpatch-through: 9.6 where this code was introduced
Discussion: https://postgr.es/m/86137f17-1dfb-42f9-7421-82fd786b04a1@anayrat.info

Branch
------
REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ba10eaef509bf47de743589a232e83f071acf8dc

Modified Files
--------------
src/backend/executor/execMain.c  | 16 ++++++++++++----
src/backend/executor/nodeLimit.c | 10 ++++++++--
2 files changed, 20 insertions(+), 6 deletions(-)