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.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-

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/9d3ebba729ebaf5882a92f0f5f662a3312037605

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