EZP-22966: Extracted DFS DFSBackend Interface

It is extracted from the current eZDFSFileHandlerDFSBackend,
that now implements it.

dfscleanup.php had to be modified as it was using quite a lot of
the eZDFSFileHandlerDFSBackend internals. New methods were created
in the interface to account for those changes.
Merge pull request #984 from ezsystems/EZP-22966-dfsbackend_interface

Extracted eZDFSFileHandlerDFSBackendInterface