node/chainstate: Decouple from concept of NodeContext

Summary:

...instead pass in only the necessary information
Also allow mempool to be a nullptr

Partial backport of core#23280:
https://github.com/bitcoin/bitcoin/pull/23280/commits/9162a4f93ef5aeb57fe11a6e09f5881cf431f5e6

Depends on D12565.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12566