[avalanche] Restrict scope of cs_stakingRewards lock during cleanup

Summary: Release the lock when we're done with it. This helps avoid lock order issues.

Test Plan:

cmake -GNinja .. -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_BUILD_TYPE=Debug
ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17336