Refactor Iroha - start Torii with all other tasks

[0x009922]

          I feel strange that torii is used to "block" iroha.

Imo it would be simpler design if iroha would always spawn torii in sparate task as any other service.

And on iroha start we would just listen on notify_sender:

async fn start(self) {
    self.notify_sender.notified().await    
}

This way iroha user is free to either block on this and wait on shutdown or start it as task.

Originally posted by @Erigara in #4456 (comment)