[avalanche] Return a specific vote value for conflicting txs

Summary:
So we can distinguish from other invalid txs.

Depends on D16645 and D16648.

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