p2p, rpc: address relay fixups

Summary:
Move a use of statestats in getpeerinfo to within the section guarded by if (fStateStats), e.g. PeerManagerImpl::GetNodeStateStats true, and pass an in-param by reference to const.

This is a backport of core#22616

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13348