[avalanche] Process responses to stake contender polls

Summary:
Implement processing of avalanche polls for stake contenders, allowing them to be finalized/invalidated and update staking rewards accordingly.

Depends on D17710

Test Plan:

ninja check-functional

Make sure the test is not flaky (making it robust was a challenge):

for I in {0..20}; do ./test/functional/test_runner.py abc_p2p_avalanche_contender_voting || break ; done

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17700