OpenTSDB: Fixed the bug that failed to write a cache data file when cache directories were removed. (https://rbcommons.com/s/pepperdata/r/2991)
- The bug was that OpenTSDB failed to create a cached data file when the
corresponding cache directory was deleted while the cache entry was cached in
memory. - The fix was to let a query miss the cache if the entry file doesn't
exist and to always ensure the cache directory whenever a new cache entry is
created.