node/ifaces: ChainImpl: Use existing NodeContext member

Summary:
This is a backport of core#21270 [9/12] and core#21525 [10/10]
https://github.com/bitcoin/bitcoin/pull/21270/commits/91c5b68acd12cf7c2b4888d54d8fdd21837b2817
https://github.com/bitcoin/bitcoin/pull/21525/commits/693414d27181cf967f787a2ca72344e52c58c7f0

Depends on D11424

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11425