Capture the Deferred for request cancellation in _AsyncResource (#12694)

All async request processing goes through `_AsyncResource`, so this is
the only place where a `Deferred` needs to be captured for cancellation.

Unfortunately, the same isn't true for determining whether a request
can be cancelled. Each of `RestServlet`, `BaseFederationServlet`,
`DirectServe{Html,Json}Resource` and `ReplicationEndpoint` have
different wrappers around the method doing the request handling and they
all need to be handled separately.

Signed-off-by: Sean Quah <seanq@element.io>

synapse/http/server.py

@@ -344,7 +344,9 @@ class _AsyncResource(resource.Resource, metaclass=abc.ABCMeta):
 
     def render(self, request: SynapseRequest) -> int:
         """This gets called by twisted every time someone sends us a request."""
-        defer.ensureDeferred(self._async_render_wrapper(request))
+        request.render_deferred = defer.ensureDeferred(
+            self._async_render_wrapper(request)
+        )
         return NOT_DONE_YET
 
     @wrap_async_request_handler