[avalanche] Make isConflictingProofPreferred a comparator

Summary:
This was intended to be a peer manager static method, but it will eventually be used in the orphan proof pool as well, so let make it a proper comparator.

Ref T1854.

Supersedes D10404.
Depends on D10435.

Test Plan:

ninja all check-avalanche

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10439