fix: allow a single node to activate stress relief mode during significant load increase

[VinozzZ]

Which problem is this PR solving?

Without resolving trace locality issue, a single peer can receive a large trace that significantly raise its stress level than the rest of the cluster. To address this issue, we can allow individual refineries to go into stress relief mode if their own stress is too high, even if the cluster's isn't.

Short description of the changes

• use the max of the individual stress level and the cluster stress level as the overall stress level when calculating stress relief activation and deactivation
• record the stress level that determined stress relief activation as stress_level metric
• add tests

[kentquirk]

This is great, and it also preserves our previous stress_level metric name, which is a nice bonus.

I made one suggestion that we add all levels to the single log message.