[avalanche] Return a specific vote value for conflicting txs
Summary:
So we can distinguish from other invalid txs.
Test Plan:
./test/functional/test_runner.py abc_p2p_avalanche_transaction_voting
Reviewers: #bitcoin_abc, bytesofman
Reviewed By: #bitcoin_abc, bytesofman
Differential Revision: https://reviews.bitcoinabc.org/D16649