[avalanche] Create a structure to accumulate avalanche votes

Summary:
This is slightly different from what is described in the avalanche paper as it uses a rolling quorum instead of rounds, which means we need to use significantly higher confidence parameter, as it increases much faster.

The vote is considered inconclusive if more than one respondant disagrees with the quorum of 8.

Confidence required for finalization is 128.

Test Plan: Added test cases.

Reviewers: #bitcoin_abc, schancel, Fabien

Reviewed By: #bitcoin_abc, schancel, Fabien

Subscribers: Fabien, jasonbcox, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2040