Pass mempool pointer to GetCoinsCacheSizeState

Summary:
This concludes backport of core#19604 [3/3]
https://github.com/bitcoin/bitcoin/pull/19604/commits/fae8c28dae747f9c4c6481049742346d18202fc8

Depends on D9757

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9758