importer: fix replication (as source) once import is finished

This fixes up commit be839e92.

neo/storage/database/importer.py

@@ -509,6 +509,7 @@ class ImporterDatabaseManager(DatabaseManager):
             " your configuration to use the native backend and restart.")
         self._import = None
         for x in """getObject getReplicationTIDList getReplicationObjectList
+                    _fetchObject
                  """.split():
             setattr(self, x, getattr(self.db, x))
 
@@ -674,6 +675,9 @@ class ImporterDatabaseManager(DatabaseManager):
                                                    length, partition)
         return r
 
+    def _fetchObject(*_):
+        raise AssertionError
+
     def getObjectHistory(self, *args, **kw):
         raise BackendNotImplemented(self.getObjectHistory)