chore: move evictOldStreams into separate method to use defer on mutex

[grobinson-grafana]

What this PR does / why we need it:

This commit splits evictOldStreams into evictOldStreams and evictOldStreamsPeriodic. This will all us to test evictOldStreams and also use defer to release the mutex.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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