public void forward(Routable routable) {
        if (routable instanceof Reply reply) {
            reply.popHandler().handleReply(reply);
        } else {
            routable.pushHandler(this);
            mbus.handleMessage((Message) routable);
        }
    }
Forwards a routable to the next hop in its route. This method will never block.
@param routable the routable to forward.
messagebus/src/main/java/com/yahoo/messagebus/IntermediateSession.java