Add FileStore.purge_tmp() method

[Jason Gerard DeRose]

To add a file to a FileStore, it's first copied to a temporary location in .dmedia/tmp/. After the content-hash is computed, the file is moved to its canonical location in .dmedia/files/.

Because os.rename() is atomic on POSIX systems, files are always in a well-defined state as either being temporary or being stored, with no fuzzy middle area. Because of this, we're treating anything in .dmedia/tmp/ as fair game for deletion.

There are a number of ways in which you might get stale temporary files: you abort an import, an error occurs during an import, a render fails, etc. So it's a good idea for us to periodically purge stale temporary files so they don't build up over time. Dmedia will do this when a FileStore is first connected.