Allow NilmDB.close to be called twice

nilmdb/server/nilmdb.py

@@ -149,6 +149,7 @@ class NilmDB(object):
         if self.con:
             self.con.commit()
             self.con.close()
+            self.con = None
         self.data.close()
 
     def _sql_schema_update(self):