R1:682c3bc9ee56

When migrating files between storage engines with "bin/files migrate ...", skip expired temporary files

Summary:
See T7148. This just cheats us out of a weird sort of race where we:

- Dump an instance, including some `F123` which is a temporary file which expires in 3 minutes.
- A few minutes later, the daemons delete the data for that file.
- A few minutes after that, we try to `bin/files migrate --copy` to copy the data from S3 into the MySQL blob store.
- This fails since the data is already…