[avalanche] Add debug logging when local contenders are set

Summary: This makes it easy to compare behavior via logs from multiple nodes with less noise.

Test Plan:

./test/functional/test_runner.py --nocleanup abc_p2p_avalanche_contender_voting

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17905