Modify DataStore.cleanup() to minimize lock hold duration:
- Snapshot timestamps under lock (brief O(1) list copy)
- Compute expired keys outside lock (no contention during O(n) scan)
- Re-acquire lock only for deletion with re-validation
(ensures entries refreshed between snapshot and deletion are not deleted)
This reduces blocking of reader threads and prevents latency spikes
during periodic cleanup of large stores (10K+ entries).
Also adds tests:
- test_cleanup_removes_expired_keeps_fresh: basic cleanup behavior
- test_cleanup_does_not_delete_refreshed_entry: re-validation guard
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
James Smith
646eb09e1d