kernel: Move non-kernel db cache size constants

Summary:
These have nothing to do with the txdb, so move them out and into the
node caches.

This is a backport of core#31483
https://github.com/bitcoin/bitcoin/pull/31483/commits/8826cae285490439dc1f19b25fa70b2b9e62dfe8
Depends on D18581

Note: the renaming of constants was already done in D8318. It's a good idea to keep the _MB suffix for now, they will be removed in a later commit when units are converted to bytes.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18582