node/caches: Extract cache calculation logic

Summary:
Partial backport of core#23280:
https://github.com/bitcoin/bitcoin/pull/23280/commits/ac4bf138b849a8544798f3891d6623803040c265

Depends on D12579.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12580