Use rust stable 1.31 instead of rust nightly for electrs, update electrs, specify electrum-rpc

modules/nix-bitcoin.nix

@@ -17,7 +17,6 @@ let
     lightning-charge
     nanopos
     spark-wallet
-    electrs
     nodejs-8_x
     nginx
   ];
@@ -175,10 +174,11 @@ in {
       version = 3;
     };
     services.electrs.enable = false;
+    services.electrs.port = 50001;
     services.electrs.high-memory = false;
     services.tor.hiddenServices.electrs = {
       map = [{
-        port = 50001; toPort = 50001;
+        port = config.services.electrs.port; toPort = config.services.electrs.port;
       }];
       version = 3;
     };