Fix assertion to stop transaction queue getting wedged

... and update some docstrings to correctly reflect the types being used.

get_new_device_msgs_for_remote can return a long under some circumstances,
which was being stored in last_device_list_stream_id_by_dest, and was then
upsetting things on the next loop.

synapse/replication/slave/storage/_slaved_id_tracker.py

@@ -27,4 +27,9 @@ class SlavedIdTracker(object):
         self._current = (max if self.step > 0 else min)(self._current, new_id)
 
     def get_current_token(self):
+        """
+
+        Returns:
+            int
+        """
         return self._current