Removed the singleton from DFSBackend factory

The DBBackend already has a singleton.
dfscleanup.php also uses it, but it is a stateless class anyway,
and it should not be a problem if it is instantiated more than once.
Added a resetDBBackend() method to eZDFSFileHandler

Makes it possible to reload the backends from configuration.
Required to override those from the new stack.
Added support for services to DFSBackend factory