Introduce stateVersion concept

With upcoming changes to the dovecot home and maildirectories we need to
introduce a way to nudge users to inform themselves about manual
migration steps they might need to carry out.

The idea here is to allow us to safely make breaking changes and notify
the user of required migration steps at eval time, so they can make the
necessary changes in time.

tests/lib/config.nix

@@ -1,3 +1,7 @@
 {
-    security.dhparams.defaultBitSize = 2048; # minimum size required by dovecot
+  # Testing eval failures that result from stateVersion assertion is out of scope
+  mailserver.stateVersion = 999;
+
+  # minimum size required by dovecot
+  security.dhparams.defaultBitSize = 2048;
 }

tests/minimal.nix

@@ -18,7 +18,10 @@
   name = "minimal";
 
   nodes.machine = {
-    imports = [  ./../default.nix ];
+    imports = [
+      ../default.nix
+      ./lib/config.nix
+    ];
   };
 
   testScript = ''

tests/multiple.nix

@@ -16,7 +16,10 @@ let
     password = pkgs.writeText "password" "password";
 
     domainGenerator = domain: { pkgs, ... }: {
-      imports = [../default.nix];
+      imports = [
+        ../default.nix
+        ./lib/config.nix
+      ];
       environment.systemPackages = with pkgs; [ netcat ];
       virtualisation.memorySize = 1024;
       mailserver = {