Delayed/recurring messages are not delivered to busy actors

[strohel]

When a Timed<> actor does something like

fn handle(&mut self, context, message) {
    sleep(some time);
    context.myself.send_now(some message);
}

Then delayed or recurring messages are never delivered to it (no matter what priority they have), because Timed wrapper only processes them in deadline_exceeded(), which is never called (because the queue in never empty in actors event loop handling).