refactor, net: Relocate CNode private members and increase data member encapsulation

Summary:
All protected CNode data members could be private.

This is a backport of core#20373

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13274