Inspect missing cache files and fix lock races

[adamnovak]

This adds code for dumping information about the cache database state if #5084 happens.

I tried to trigger it somehow by running the caching filestore tests locally, and I found a different bug, which is a race in a bunch of places we use safe_lock(), because safe_lock() makes no guarantees that the file you locked is still the one at the path you opened. We already handled this in the mutex implementation, but we didn't handle this in other places where the file might be going away or being replaced.

So this fixes that problem. I still haven't been able to actually trigger #5084 on my machine though.

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