validation: Pass in chainstate to ::LoadMempool

Summary:
This is a backport of core#20750 [9/17]
https://github.com/bitcoin/bitcoin/pull/20750/commits/d0da7ea57ab932eca956458fb3633585ff3c0003

Depends on D11194

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11195