Evaluate `crate::service::globals::Service::stateres_mutex`

I'm labeling this as a bug because it currently doesn't work as intended (the lock method is never awaited so a lock guard is never acquired). We should:

1. Fix it
2. Measure the hits and misses on this cache¹ (see also #13 (closed))
3. On a branch, remove it and measure again
4. Decide whether to leave it fixed or remove it based on the results

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1. @Lambda suggested this was relevant to stateres_mutex but I haven't tried to verify this myself. The metrics will tell us for sure though. ↩