[avalanche] Streamline API for setting manual stake winners to contender cache

Summary:
This more closely matches Processor::setStakingRewardWinners which will help eliminate unnecessary logic that loops over new winners to add them to the cache. When the cache is ready to be populated, this line can be dropped into setStakingRewardWinners() with minimal other changes:

stakeContenderCache.setWinners(pprev, payouts);

Test Plan:

ninja check-avalanche-stakecontendercache_tests

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16933