pgsql: Fix leaked hash scans in pg_listening_channels().

Fix leaked hash scans in pg_listening_channels().

Commit 282b1cde9 changed pg_listening_channels() to keep a hash table
scan open between calls of this set-returning function. If the
executor stops fetching rows early, as with a target-list call under
LIMIT, the scan remains registered until transaction end, resulting in
leak complaints and eventually a PANIC.

Instead materialize the result in a tuplestore, so that we can
complete the hash scan within a single call.

Author: Joel Jacobson <joel(at)compiler(dot)org>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/069fc976-bee1-4f0f-b22b-86ef7ed4f9ff@app.fastmail.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/2b491ff1cd2e23ac7a9f9efbd59ddd4aa02dbb58

Modified Files
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src/backend/commands/async.c | 56 ++++++++++++++-----------------------
src/test/regress/expected/async.out | 7 +++++
src/test/regress/sql/async.sql | 4 +++
3 files changed, 32 insertions(+), 35 deletions(-)