pgsql: Refactor some repetitive SLRU code

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c61678551699610d658edb0ae838d2541688caba

Modified Files
--------------
src/backend/access/transam/clog.c       |  67 ++----------------
src/backend/access/transam/commit_ts.c  |  66 +++---------------
src/backend/access/transam/multixact.c  | 120 ++++----------------------------
src/backend/access/transam/slru.c       |  25 +++++++
src/backend/access/transam/subtrans.c   |  34 ++-------
src/backend/access/transam/xloginsert.c |  12 ++++
src/include/access/slru.h               |   1 +
src/include/access/xloginsert.h         |   1 +
8 files changed, 71 insertions(+), 255 deletions(-)