[TECH-SUPPORT] LPS-36609 Migration from FileSystemStore to AdvancedFileSystemStore doubles filesize usage

Hey Sergio,

Originally LPS-36609 used to be a feature request, but I had to convert it to a bug, because I'm working a customer who is actually facing this problem.

Even though the control panel offers the migration to AFSS, it's no way to specify the path where you'd like to migrate onto. Consequently when you do the actual convert it causes two problems:

* Doubles the DL store size
* It doesn't clean up the old files

Thanks for reviewing!

Cheers,
László