pgsql: Add PgStat_KindInfo.init_shmem_cb

Add PgStat_KindInfo.init_shmem_cb

This new callback gives fixed-numbered stats the possibility to take
actions based on the area of shared memory allocated for them.

This removes from pgstat_shmem.c any knowledge specific to the types
of fixed-numbered stats, and the initializations happen in their own
files.  Like b68b29bc8fec, this change is useful to make this area of
the code more pluggable, so as custom fixed-numbered stats can take
actions after their shared memory area is initialized.

Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/Zot5bxoPYdS7yaoy@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/21471f18e9d9aa3383314ed8bc9163bc369109ae

Modified Files
--------------
src/backend/utils/activity/pgstat.c              |  6 ++++++
src/backend/utils/activity/pgstat_archiver.c     |  8 ++++++++
src/backend/utils/activity/pgstat_bgwriter.c     |  8 ++++++++
src/backend/utils/activity/pgstat_checkpointer.c |  8 ++++++++
src/backend/utils/activity/pgstat_io.c           |  9 +++++++++
src/backend/utils/activity/pgstat_shmem.c        | 21 +++++++++++----------
src/backend/utils/activity/pgstat_slru.c         |  8 ++++++++
src/backend/utils/activity/pgstat_wal.c          |  8 ++++++++
src/include/utils/pgstat_internal.h              | 13 +++++++++++++
9 files changed, 79 insertions(+), 10 deletions(-)