Improve reliability of offline sync

Reliability of the offline sync could use some improvements - especially with larger files. Sometimes it can stop the transfer at 40% and then report the file as successfully synced.

In the image attached you can see one of the examples of corrupted transfer - the synced file plays in the Emby-UWP, but the seeking doesn't work. The app and the server are running on the same machine, so it's not a network issue.