rpc/blockchain: Use existing NodeContext

Summary:
Also pass in appropriate object to: BIP9SoftForkDescPushBack

PR context: De-globalize ChainstateManager

This is a backport of core#21391
https://github.com/bitcoin/bitcoin/pull/21391/commits/d485e815e2b62dc74a485569d08130dc3ef9ff63

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11510