Improved validation for received requests (#9817)

* Simplify `start_listening` callpath

* Correctly check the size of uploaded files

synapse/server.py

@@ -287,6 +287,14 @@ class HomeServer(metaclass=abc.ABCMeta):
         if self.config.run_background_tasks:
             self.setup_background_tasks()
 
+    def start_listening(self) -> None:
+        """Start the HTTP, manhole, metrics, etc listeners
+
+        Does nothing in this base class; overridden in derived classes to start the
+        appropriate listeners.
+        """
+        pass
+
     def setup_background_tasks(self) -> None:
         """
         Some handlers have side effects on instantiation (like registering