[lib] add scheduler to node context

Summary:

• also update test setup & access point in denial of service test

This is a partial backport of Core PR18037 : https://github.com/bitcoin/bitcoin/pull/18037/commits/7c8b6e5b5206a98f86675d0107ad99ea1d080466

Depends on D6501

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6502