Calculate memory usage correctly for unordered_maps that use PoolAllocator

Summary:
Extracts the resource from a PoolAllocator and uses it for
calculation of the node's memory usage.

This is a partial backport of core#25325
https://github.com/bitcoin/bitcoin/pull/25325/commits/e19943f049ed8aa4f32a1d8440a9fbf160367f0f

with an ARM fix and additional test to make sure the pool is actually used by the nodes from core#28913

Depends on D16175

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16176