Remove mempool global from interfaces

Summary:

The chain interface has an m_node member, which has a pointer to the mempool global. Use the pointer instead of the global to prepare the removal of the mempool global. See core#19556

This is a backport of core#19848

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9784