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/storage/util/id_generators.py

@@ -30,6 +30,17 @@ class IdGenerator(object):
 
 
 def _load_current_id(db_conn, table, column, step=1):
+    """
+
+    Args:
+        db_conn (object):
+        table (str):
+        column (str):
+        step (int):
+
+    Returns:
+        int
+    """
     cur = db_conn.cursor()
     if step == 1:
         cur.execute("SELECT MAX(%s) FROM %s" % (column, table,))
@@ -131,6 +142,9 @@ class StreamIdGenerator(object):
     def get_current_token(self):
         """Returns the maximum stream id such that all stream ids less than or
         equal to it have been successfully persisted.
+
+        Returns:
+            int
         """
         with self._lock:
             if self._unfinished_ids: