[avalanche] Use block index instead of blockhash for adding contenders to the cache

Summary: This will facilitate adding block height to the cache per feedback in D16782. While the existing API is not cumbersome in practice, it is more error prone when writing tests.

Test Plan:

ninja check-avalanche

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16793