Improved validation for received requests (#9817)

* Simplify `start_listening` callpath

* Correctly check the size of uploaded files

tests/test_server.py

@@ -202,6 +202,7 @@ class OptionsResourceTests(unittest.TestCase):
             parse_listener_def({"type": "http", "port": 0}),
             self.resource,
             "1.0",
+            max_request_body_size=1234,
             reactor=self.reactor,
         )