[avalanche] Poll for stake contenders

Summary:
Poll for stake contenders after they have been promoted to a new chain tip.

Responses to polls will be handled in a follow up diff.

Depends on D17615

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17607