Fix memory accumulation in pg_sync_replication_slots() during retries.

Fix memory accumulation in pg_sync_replication_slots() during retries.

Unlike the slotsync worker, whose retry cycles are separated by
transaction boundaries, pg_sync_replication_slots() retries within a
single SQL function call. Per-cycle allocations for slot names, plugin
names, database names, and auxiliary list containers get accumulated
across retries until the function returned. Memory growth is proportional
to the number of retries and remote slots, and the function may wait an
extended period between cycles when slots are slow to persist.

Fix by running each retry cycle in a short-lived memory context
(sync_retry_ctx) that is reset before the next attempt. Additionally,
release tuple slots created with MakeSingleTupleTableSlot() before
clearing the walreceiver result.

Author: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Zhijie Hou <houzj.fnst@fujitsu.com>
Reviewed-by: shveta malik <shveta.malik@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/CABPTF7VVPxgfYyr8Kyi=+JACjckQ6NpniV9eRtHboj2hMn0REw@mail.gmail.com