[avalanche] Add some logging when stake contenders are cached or promoted

Summary:
These logs are only active with -avalanchestakingpreconsensus

I have been using this patch for over a week and it has been useful for debugging contender behavior.

Test Plan:

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

Observe new lines in the node log

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17860