[avalanche] Avoid O(n2) when voting on proofs

Summary:
This fix the quadratic complexity that occurs when voting on proofs. There is no change in behavior.

Ref T1854.

Depends on D10378 and D10384.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10379