Need a delegate callback to respond to upload/download operation progress

Dropbox has a nice delegate callback that provides upload and download progress updates during the operation. For larger file transfers, this is especially helpful because you can provide a progress bar for the user rather than them just watching a spinning activity indicator. 

So, we piggy backed on the framework again to create App Sync Manager and Doc Sync Manager delegate methods to respond to these Dropbox progress updates. They are optional delegates so are not required to be used, but definitely nice to have.

Also, this is only implemented for the Dropbox mode. I'm not sure if the FileSystem and/or iCloud modes have similar delegate callbacks. If they do, they could be easily wired into the code we've added here.