pgsql: Use atomic access for SlruShared->latest_page_number

Use atomic access for SlruShared->latest_page_number

The new concurrency model proposed for slru.c to improve performance
does not include any single lock that would coordinate processes
doing concurrent reads/writes on SlruShared->latest_page_number.
We can instead use atomic reads and writes for that variable.

Author: Dilip Kumar <dilipbalaut(at)gmail(dot)com>
Reviewed-by: Andrey M. Borodin <x4mmm(at)yandex-team(dot)ru>
Discussion: https://postgr.es/m/CAFiTN-vzDvNz=ExGXz6gdyjtzGixKSqs0mKHMmaQ8sOSEFZ33A@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d172b717c6f436738cc8383a4e9f611ae227fd93

Modified Files
--------------
src/backend/access/transam/clog.c | 6 +-----
src/backend/access/transam/commit_ts.c | 7 +++---
src/backend/access/transam/multixact.c | 28 ++++++++++++++----------
src/backend/access/transam/slru.c | 39 ++++++++++++++++++++++++----------
src/include/access/slru.h | 5 ++++-
5 files changed, 53 insertions(+), 32 deletions(-)