[avalanche] Keep track how how many votes occured.

Summary: As per title. This helps keeping track of statistics if we want in the future, but mostly to be able to keep track of information about the last n voters in a circular bffr in the future.

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2583