Save dc_id instead layer and salt in the session file

Server salts change every 30 minutes after all, so keeping them
in the long-term storage session file doesn't make much sense.

Saving the layer doesn't make sense either, as it was only used
to know whether to init connection or not, but it should be done
always.

telethon_tests/higher_level_test.py

@@ -18,7 +18,7 @@ class HigherLevelTests(unittest.TestCase):
     @staticmethod
     def test_cdn_download():
         client = TelegramClient(None, api_id, api_hash)
-        client.session.server_address = '149.154.167.40'
+        client.session.set_dc(0, '149.154.167.40', 80)
         assert client.connect()
 
         try: