electrs: adapt to version 0.9.0

- `waitfornewblock` was previously not included in the public RPC
  whitelist because it's reserved for testing and marked as hidden
  in bitcoind.

- electrs changed its verbosity settings. `-vv` is now the best choice
  for normal usage.

- bitcoind option `dataDirReadableByGroup` is now unused.
  Because it can be valuable for other use cases and implementing
  it is intricate, we're keeping it for now.

- test: keep `nc` connection open because otherwise the electrs
  RPC server would now close the connection before sending a response.

test/tests.nix

@@ -74,6 +74,8 @@ let
       tests.charge-lnd = cfg.charge-lnd.enable;
 
       tests.electrs = cfg.electrs.enable;
+      # Sigterm is broken during IBD in version 0.9.0 https://github.com/romanz/electrs/issues/532
+      systemd.services.electrs.serviceConfig.KillSignal = "SIGKILL";
 
       tests.liquidd = cfg.liquidd.enable;
       services.liquidd.extraConfig = mkIf config.test.noConnections "connect=0";