refactor: Do not expose CAddrMan members as protected without need

Summary:
Partial backport of core#22025:
https://github.com/bitcoin/bitcoin/pull/22025/commits/5cd7f8abe3996d303774b6cddeb419337d605d02

Actually this backport does a bit more than the source material because:

1. There is no point adding the private label for a class, it's already the default;
2. Our members visibility are all over the place for some reason, so I better fix it completely.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12253