[net/refactor] Remove m_manual_connection flag from CNode

Summary:
Partial backport (10/19) of core PR19316:
https://github.com/bitcoin/bitcoin/pull/19316/commits/49efac5cae7333c6700d9b737d09fae0f3f4d7fa

Depends on D8719.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8720