'This method is called by the server when a new request has started.
:arg server_conn: is an opaque object representing the long-lived
(e.g. tcp-level) connection.
:arg request_conn: is a `.HTTPConnection` object for a single
request/response exchange.
This method should return a `.HTTPMessageDelegate`.'

def start_request(self, server_conn, request_conn):

    raise NotImplementedError()
