LPS-48961 use DLFolder instance instead of Folder

At this point in the code it is known that the folder is a DLFolder. By
using DLFolder directly we'll avoid exposing the restriction type in the
Folder interface.
LPS-48961 don't expose restriction type

Folder is used by all kinds of repositories (e.g. Documentum,
Sharepoint, etc), where restriction types don't make sense.
LPS-48961 Fix compilation issues