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 b68b29bc8f, 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

src/backend/utils/activity/pgstat_archiver.c

@@ -62,6 +62,14 @@ pgstat_fetch_stat_archiver(void)
 	return &pgStatLocal.snapshot.archiver;
 }
 
+void
+pgstat_archiver_init_shmem_cb(void *stats)
+{
+	PgStatShared_Archiver *stats_shmem = (PgStatShared_Archiver *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_archiver_reset_all_cb(TimestampTz ts)
 {