pgstat: Create memory contexts below TopMemoryContext

So far they were created below CacheMemoryContext. However, that's not
guaranteed to exist in all situations, leading to memory contexts created as
top-level contexts. There isn't actually a good reason anymore to create them
below CacheMemoryContext, so just creating them below TopMemoryContext seems
the best approach.

Reported-by: Reid Thompson <reid.thompson@crunchydata.com>
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Author: "Drouvot, Bertrand" <bdrouvot@amazon.com>
Discussion: https://postgr.es/m/b948b729-42fe-f88c-2f4a-0e65d84c049b@amazon.com
Backpatch: 15-

src/backend/utils/activity/pgstat.c

@@ -1069,7 +1069,7 @@ pgstat_prep_pending_entry(PgStat_Kind kind, Oid dboid, Oid objoid, bool *created
 	if (unlikely(!pgStatPendingContext))
 	{
 		pgStatPendingContext =
-			AllocSetContextCreate(CacheMemoryContext,
+			AllocSetContextCreate(TopMemoryContext,
 								  "PgStat Pending",
 								  ALLOCSET_SMALL_SIZES);
 	}

src/backend/utils/activity/pgstat_shmem.c

@@ -992,12 +992,12 @@ pgstat_setup_memcxt(void)
 {
 	if (unlikely(!pgStatSharedRefContext))
 		pgStatSharedRefContext =
-			AllocSetContextCreate(CacheMemoryContext,
+			AllocSetContextCreate(TopMemoryContext,
 								  "PgStat Shared Ref",
 								  ALLOCSET_SMALL_SIZES);
 	if (unlikely(!pgStatEntryRefHashContext))
 		pgStatEntryRefHashContext =
-			AllocSetContextCreate(CacheMemoryContext,
+			AllocSetContextCreate(TopMemoryContext,
 								  "PgStat Shared Ref Hash",
 								  ALLOCSET_SMALL_SIZES);
 }