[avalanche] Clarify and simplify checking polled contenders

Summary:
The current implementation is unnecessarily verbose and spreads out the accounting, making the intent less clear.

This patch makes it more clear and concise.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_contender_voting

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17699