Additional type hints for config module. (#11465)

This adds some misc. type hints to helper methods used
in the `synapse.config` module.

synapse/app/_base.py

@@ -32,6 +32,7 @@ from typing import (
     Iterable,
     List,
     NoReturn,
+    Optional,
     Tuple,
     cast,
 )
@@ -129,7 +130,7 @@ def start_worker_reactor(
 def start_reactor(
     appname: str,
     soft_file_limit: int,
-    gc_thresholds: Tuple[int, int, int],
+    gc_thresholds: Optional[Tuple[int, int, int]],
     pid_file: str,
     daemonize: bool,
     print_pidfile: bool,