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.

modules/versioning.nix

@@ -138,6 +138,37 @@ let
         [1] https://github.com/JoinMarket-Org/joinmarket-clientserver/blob/master/docs/fidelity-bonds.md
       '';
     }
+    {
+      version = "0.0.53";
+      condition = config.services.electrs.enable;
+      message = let
+        dbPath = "${config.services.electrs.dataDir}/mainnet";
+      in ''
+        Electrs 0.9.0 has switched to a new, more space efficient database format,
+        reducing storage demands by ~60% [1].
+        When started, electrs will automatically reindex the bitcoin blockchain.
+        This can take a few hours, depending on your hardware. The electrs server is
+        inactive during reindexing.
+
+        To upgrade, do the following:
+
+        - If you have less than 40 GB of free space [2] on the electrs data dir volume:
+          1. Delete the database:
+             systemctl stop electrs
+             rm -r '${dbPath}'
+          2. Deploy the new system config to your node
+
+        - Otherwise:
+          1. Deploy the new system config to your node
+          2. Check that electrs works as expected and delete the old database:
+             rm -r '${dbPath}'
+
+        [1] https://github.com/romanz/electrs/blob/557911e3baf9a000f883a6f619f0518945a7678d/doc/usage.md#upgrading
+        [2] This is based on the bitcoin blockchain size as of 2021-09.
+            The general formula is, approximately, size_of(${dbPath}) * 0.6
+            This includes the final database size (0.4) plus some extra storage (0.2).
+      '';
+    }
   ];
 
   mkOnionServiceChange = service: {