Allow Payer HA

[mkysel]

          I think given the caveats in the description this is fine.

We may have to think through a more complete nonce management strategy that coordinates between multiple instances of the payer service.

But for right now we can recommend just having a single instance

Originally posted by @neekolas in #330 (review)

[mkysel]

Because of the nonce management, we can't have multiple payer instances. If we do, they will collide. This is OK for the time being, but it needs a proper fix.