init: Use size_t consistently for cache sizes

Summary:
This avoids having to rely on implicit casts when passing them to the
various functions allocating the caches.

This also ensures that if the requested amount of db_cache does not fit
in a size_t, it is clamped to the maximum value of a size_t.

Also take this opportunity to make the total amounts of cache in the
chainstate manager a size_t too.

This concludes backport of core#31483
https://github.com/bitcoin/bitcoin/pull/31483/commits/2a92702bafca5c78b270a9502a22cb9deac02cfc
Depends on D18583

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18584