[avalanche] Make sure we keep the read lock when reading from VoteRecord

Summary: At least two function read from VoteRecord after releasing the read lock from vote_records, which is incorrect.

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2470