pgsql: Fix fetching default toast value during decoding of in-progress

Fix fetching default toast value during decoding of in-progress transactions.

During logical decoding of in-progress transactions, we perform the toast
table scan while fetching the default toast value for an attribute. We
forgot to initialize the flag during this scan to indicate that the system
table scan is in progress. We need this flag to ensure that during logical
decoding we never directly access the tableam or heap APIs because we check
for concurrent aborts only in systable_* APIs.

Reported-by: Alexander Lakhin
Author: Takeshi Ideriha, Hou Zhijie
Reviewed-by: Amit Kapila, Hou Zhijie
Backpatch-through: 14
Discussion: https://postgr.es/m/18641-6687273b7f15269d@postgresql.org

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8175a7d11f70589f6b44e28a8dd804f7542d0bbf

Modified Files
--------------
contrib/test_decoding/expected/stream.out | 20 ++++++++++++++++++++
contrib/test_decoding/sql/stream.sql      | 18 ++++++++++++++++++
src/backend/access/index/genam.c          | 16 ++++++++++++++++
3 files changed, 54 insertions(+)