[avalanche] Register votes on conflicting proofs
Register the votes on conflicting proofs and return an list of the updates as consequence. The logic is the same than the block, as is demonstrated by the unit tests being compatible with the ProofOnlyTestingContext.
ninja all check-all
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Maniphest Tasks: T1854
Differential Revision: https://reviews.bitcoinabc.org/D10271