Made DFS FS backend configurable

A new INI setting, eZDFSClusteringSettings.DFSBackend, is introduced.
It sets the backend class used by DFS.

The backend instance is provided by eZDFSFileHandlerBackendFactory.

If a backend implements eZDFSFileHandlerFactoryDFSBackendInterface,
it will be constructed using a 'factory()' static method. If it doesn't,
and instance will be created using 'new'.