[avalanche] Eliminate taking unnecessary locks on cs_stakingRewards when getting stake contender status

Summary: It is unnecessary to take a lock on cs_stakingRewards unless there is a cache hit. There is no change in behavior.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17616