[avalanche] Reduce verbosity of the staking rewards preconsensus in logs

Summary: Most of the logs are useful for debugging but not desirable for normal logging. Let's reduce their log level to trace so it's opt-in.

Test Plan:

ninja check-functional

Check the logs are gone unless -loglevel=trace is set.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18719