[net processing] Add Peer& arg to MaybeDiscourageAndDisconnect()

Summary:
Refactor only. No change in behaviour.

Partial backport of core#20721:
https://github.com/bitcoin/bitcoin/pull/20721/commits/f8b3058992b507f3a6aac9d4e2db00102ae1b197

Ref T1696.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1696

Differential Revision: https://reviews.bitcoinabc.org/D10873