[avalanche] Add a facility to replace a proof in the pool

Summary:
This is selected according to the ConflictingProofComparator(). The previous addProof() function keep the previous behavior and is renamed to make the intent clearer. This will be used to proceed to proof replacement in future diffs. There is no change in behavior.

Ref T1854.

Test Plan:

ninja all check-avalanche

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10651