[avalanche] Ensure every connected block has staking rewards computed

Summary: While staking rewards is robust enough to skip computing, it is not the expected behavior.

Test Plan:

ninja check check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17306