Handle async cancellation in select

[voidc]

Certain uses of the select macro do not handle async cancellation correctly which may lead to data loss.
(also see https://blog.yoshuawuyts.com/futures-concurrency-3/)

allium/src/onion/circuit.rs

Lines 165 to 172 in ea308e6

	tokio::select! {
	msg = self.in_circuit.accept_opaque() => self.handle_in_circuit(msg).await?,
	msg = out_circuit.accept_opaque() => self.handle_out_circuit(msg).await?,
	_ = &mut delay => {
	self.handle_timeout().await;
	break;
	},
	}

For example here, accept_opaque is not cancellation safe which causes data loss in the not-selected future.