Handle LP downtimes more gracefully

At the moment if SCA detects that LP is unavailable it will not proceed to create a payment. This is better than letting people fill out their credit card details and *then* telling them that LP is unavailable, but it still has a couple of drawbacks:
 - If LP goes down *during* a transaction s-c-a will not be able to create the subscription once the user has completed the payment, so it'll still be an awful user experience. It would be better if SCA had an MAINTENANCE_MODE setting that tells it to not start new payments. Payments already initiated can then be completed if we set SCA to maintenance mode a while before LP goes down for a rollout.
 - LP does a rollout about once a month, so we'll have a monthly downtime of about an hour or more. Ideally we wouldn't stop attending requests during that time, but we'd have some way of queuing up requests and processing them once LP is back up. Perhaps the payment notification handler could take care of this.