Do not reject INTRO_ESTABLISHED messages with extensions
The spec says that we should ignore unrecognized extensions. Per discussion at torspec#249 (closed), this is still correct.
Additionally, this commit moves responsibility for checking INTRO_ESTABLISHED messages into IptMsgHandler::handle_msg, to make sure that the circuit is torn down as soon as possible on a bad reply. (There's nothing to check yet, but there will be once we start sending extensions that expect a reply.)
Closes #1238 (closed).