pgsql: pgstat: add pgstat_copy_relation_stats().

pgstat: add pgstat_copy_relation_stats().

Until now index_concurrently_swap() directly modified pgstat internal
datastructures. That will break with the introduction of shared memory
statistics and seems off architecturally.

This is done separately from the - quite large - shared memory statistics
patch to make review easier.

Author: Andres Freund <andres@anarazel.de>
Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reviewed-By: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/20220303021600.hs34ghqcw6zcokdh@alap3.anarazel.de

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/8ea7963fc741b6f403a544d56ad0ecf78e5237b1

Modified Files
--------------
src/backend/catalog/index.c                  | 26 ++--------------------
src/backend/utils/activity/pgstat_relation.c | 32 ++++++++++++++++++++++++++++
src/include/pgstat.h                         |  2 ++
3 files changed, 36 insertions(+), 24 deletions(-)