Add start up check that mint was able to establish LN connection and is waiting for invoices

[thesimplekid]

We should a a check that the mint was able to establish a connection to the ln back and is waiting got invoiced before starting the rest of the mint.

cdk/crates/cdk-mintd/src/main.rs

Lines 362 to 374 in 7aa83bf

	tokio::spawn({
	let shutdown = Arc::clone(&shutdown);
	async move { mint.wait_for_paid_invoices(shutdown).await }
	});
	let axum_result = axum::Server::bind(
	&format!("{}:{}", listen_addr, listen_port)
	.as_str()
	.parse()?,
	)
	.serve(mint_service.into_make_service())
	.await;

[thesimplekid]

related to #506