[avalanche] Use initializer for VoteRecord
[avalanche] Use initializer for VoteRecord

Summary: It's simpler.

Test Plan:
  make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2474

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