pgsql: pgstat: Create memory contexts below TopMemoryContext

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

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/fb503793ef50eb16eb1924251d2ed58e1621eeb3

Modified Files
--------------
src/backend/utils/activity/pgstat.c | 2 +-
src/backend/utils/activity/pgstat_shmem.c | 4 ++--
2 files changed, 3 insertions(+), 3 deletions(-)