Fix the review comments and a bug in the slot sync code.

Ensure that when updating the catalog_xmin of the synced slots, it is
first written to disk before changing the in-memory value
(effective_catalog_xmin). This is to prevent a scenario where the
in-memory value change triggers a vacuum to remove catalog tuples before
the catalog_xmin is written to disk. In the event of a crash before the
catalog_xmin is persisted, we would not know that some required catalog
tuples have been removed and the synced slot would be invalidated.

Change the sanity check to ensure that remote_slot's confirmed_flush LSN
can't precede the local/synced slot during slot sync. Note that the
restart_lsn of the synced/local slot can be ahead of remote_slot. This can
happen when slot advancing machinery finds a running xacts record after
reaching the consistent state at a later point than the primary where it
serializes the snapshot and updates the restart_lsn.

Make the check to sync slots robust by allowing to sync only when the
confirmed_lsn, restart_lsn, or catalog_xmin of the remote slot is ahead of
the synced/local slot.

Reported-by: Amit Kapila and Shveta Malik
Author: Hou Zhijie, Shveta Malik
Reviewed-by: Amit Kapila, Bertrand Drouvot
Discussion: https://postgr.es/m/OS0PR01MB57162B67D3CB01B2756FBA6D94062@OS0PR01MB5716.jpnprd01.prod.outlook.com
Discussion: https://postgr.es/m/CAJpy0uCSS5zmdyUXhvw41HSdTbRqX1hbYqkOfHNj7qQ+2zn0AQ@mail.gmail.com

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