[avalanche] Merge the block and proof vote records into a single data structure

Summary:
This will prevent duplicating the code several time when adding vote records for transactions.
More cleanups and factorizations will follow.

There is no change in behavior.

Test Plan:

ninja all check-extended
../contrib/teamcity/build-configurations.py build-tsan

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12986