upgrade PaymentServer

• Prefix all the metric names with "payment_" (https://github.com/PrivateStorageio/PaymentServer/pull/98)
• Enforce a certain number of tokens on redemption (https://github.com/PrivateStorageio/PaymentServer/pull/99)
• Update the README to reflect PaymentServer's new API. (https://github.com/PrivateStorageio/PaymentServer/pull/100)

nixos/pkgs/zkapissuer/repo.nix

@@ -2,6 +2,6 @@
 fetchFromGitHub {
   owner = "PrivateStorageio";
   repo = "PaymentServer";
-  rev = "2ca189c51a5314eb476d1866d28ee483701af6f5";
-  sha256 = "1sb53lvi6alwdjhx4cv1gg7ywkqf18n1zjgdj3nigq3vjwxhdcpb";
+  rev = "ff30e85c231a3b5ad76426bbf8801f8f76884367";
+  sha256 = "1spz19f5z96shmfpazj0rv6877xvchf3gl49a4xahjbbsz39x34x";
 }