[avalanche] Implement cleanup for stake contender cache

Summary: The stake contender cache needs a way to be cleaned up so that it does not grow unbounded. The intent is to call cleanup() when a block becomes newly finalized since there is no need to maintain the cache for finalized blocks.

Test Plan:

ninja check-avalanche

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16782