[avalanche] Clarify locking expectation in GetAvalancheVoteForTx

Summary: Also release the cs_main lock when calling isPolled to avoid a lock order inversion between the RWLock for voteRecords and cs_main.

Test Plan: Run the abc_p2p_avalanche_transaction_finalization.py test with TSAN

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18513