Refine telemetry for cache walker

Part of #2292 (closed)

This change brings a few small tweaks to the cache walker to improve observability.

Two new counters are introduced to track empty directories found, and how many are successfully deleted.

A slight refactoring of the directory walker removes noise from logs and counters showing false errors.

Finally, extra precautions are taken to ensure only one cache walker goroutine is created per storage location. This prevents excessive resource consumption, but also helps clean up the noise in our logs/counters since concurrent cache walkers won't butt heads trying to delete the same directories.