[avalanche] Make it possible to replace a proof

Summary:
This lets the peer manager decide to replace a proof with another instead of rejecting the conflict.
This feature is hidden behind a flag so the behavior is not changed by default.

Ref T1854.

Depends on D10714.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix, tyler-smith

Reviewed By: #bitcoin_abc, deadalnix, tyler-smith

Subscribers: tyler-smith, deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10525