Refactor some repetitive SLRU code

Álvaro Herrera <alvherre@kurilemu.de>

Commit: c61678551699610d658edb0ae838d2541688caba
Author: Álvaro Herrera <alvherre@kurilemu.de>
Date: 2025-07-07T14:49:19Z
Refactor some repetitive SLRU code

Functions to bootstrap and zero pages in various SLRU callers were
fairly duplicative.  We can slash almost two hundred lines with a couple
of simple helpers:

 - SimpleLruZeroAndWritePage: Does the equivalent of SimpleLruZeroPage
   followed by flushing the page to disk
 - XLogSimpleInsertInt64: Does a XLogBeginInsert followed by XLogInsert
   of a trivial record whose data is just an int64.

Author: Evgeny Voropaev <evgeny.voropaev@tantorlabs.com>
Reviewed by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed by: Andrey Borodin <x4mmm@yandex-team.ru>
Reviewed by: Aleksander Alekseev <aleksander@timescale.com>
Discussion: https://www.postgresql.org/message-id/flat/97820ce8-a1cd-407f-a02b-47368fadb14b%40tantorlabs.com

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