[move only] Collect all private members of PeerLogicValidation together
Summary:
We don't have a project style for ordering class members, but it always makes sense to have no more than one of each public/protected/private specifier. Also move documentation for MaybeDiscourageAndDisconnect to the header.
Partial backport of core PR19791 (1/9):
https://github.com/bitcoin/bitcoin/pull/19791/commits/824bbd1ffba3df7ffa6f5bfaa31298cd484473b1
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D8940