refactor: promote some -netinfo localvars to class members

Summary:
This is a backport of core#20002 [4/5]
https://github.com/bitcoin/bitcoin/pull/20002/commits/82fd40216c70037480150d2b62e2b58c57784546

Depends on D10504

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10505