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>