dirty flag

[gilesknap]

Add a flag to the SyncFiles table to indicate if the media item has been synchronised to disk.

Do not attempt to download files that are marked as 'clean' (downloaded)

This will allow the local copy to be reorganized if required BUT only if the DB is kept safe, --flush-db would cause all moved/renamed files to be downloaded again.

[gilesknap]

This is done. Also:
added --retry-download to ignore the downloaded flag
and --rescan to ignore the last scan date