rpc/mining: Use existing NodeContext

Summary:
Also pass in appropriate object to:

• GetNetworkHashPS
• [gG]enerateBlock{,s}

Also:

• Misc style/constness changes

This is a partial backport of core#21391
Depends on D11507
https://github.com/bitcoin/bitcoin/pull/21391/commits/60dc05afc6f6388c6f86729a0edd7cb69f1748e0

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11511