[net/refactor] Clarify logic for selecting connections in ThreadOpenConnections

Summary:
Partial backport (14/19) of core PR19316:
https://github.com/bitcoin/bitcoin/pull/19316/commits/4972c21b671ff73f13a1b5053338b6abbdb471b5

Depends on D8723.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8724