[avalanche] Select the favorite conflicting proof

Summary:
This diff selects the favorite between 2 proofs from the conflicting pool.

Ref T1854.

Depends on D10651 and D10658.

Test Plan:

ninja all check-avalanche

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10660