Implement update sync

[demizer]

Basic sync includes getting the files on formatted devices, this is largely done. Update sync is syncing backup paths to devices that already contain files.

• A file is added
• A file is added to a directory, and the directory has previously been synced to a device that is now full.
• A file is removed from the source
• A file is removed at the destination
• A file is changed at the source
• A file exists that is read-only (0444 perms)
• Sync changed files with rsync algorithm