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: http://www.postgresql.org/message-id/flat/97820ce8-a1cd-407f-a02b-47368fadb14b%40tantorlabs.com