Check files that are not ready (`file_locations` -> `file_location_ready`) are actually OK

Ran into a bug where a file sync'ed (all jobs at 100% complete) but somehow the `file_location_ready` is left as `false`.

Have periodic checks that see if the file exists and, if so and there's no sync jobs pending, call `file_sync_shared` (or whatever tool is best) to verify the sum and marks it as ready.