Improved validation for received requests (#9817)

* Simplify `start_listening` callpath

* Correctly check the size of uploaded files

tests/replication/_base.py

@@ -359,6 +359,7 @@ class BaseMultiWorkerStreamTestCase(unittest.HomeserverTestCase):
             config=worker_hs.config.server.listeners[0],
             resource=resource,
             server_version_string="1",
+            max_request_body_size=4096,
             reactor=self.reactor,
         )