[net/refactor] Remove fInbound flag from CNode

Summary:
Partial backport (13/19) of core PR19316:
https://github.com/bitcoin/bitcoin/pull/19316/commits/60156f5fc40d56bb532278f16ce632c5a8b8035e

Depends on D8722.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8723