nixos/exim: Make queue runner interval configurable and reduce it to 5m

Exim spawns a new queue runner every n minutes as configured by the
argument to -q; up to queue_run_max can be active at the same time.
Spawning a queue runner only every 30 mins means that a message that
failed delivery on the first attempt (e.g. due to greylisting) will only
be retried 30 minutes later.

A queue runner will immediately exit if the queue is empty, so it is
more a function on how quickly Exim will scale to mail load and how
quickly it will retry than something that is taxing on an otherwise
empty system.

nixos/modules/services/mail/exim.nix

@@ -67,6 +67,13 @@ in
         '';
       };
 
+      queueRunnerInterval = mkOption {
+        type = types.str;
+        default = "5m";
+        description = ''
+          How often to spawn a new queue runner.
+        '';
+      };
     };
 
   };
@@ -104,7 +111,7 @@ in
       wantedBy = [ "multi-user.target" ];
       restartTriggers = [ config.environment.etc."exim.conf".source ];
       serviceConfig = {
-        ExecStart   = "${cfg.package}/bin/exim -bdf -q30m";
+        ExecStart   = "${cfg.package}/bin/exim -bdf -q${cfg.queueRunnerInterval}";
         ExecReload  = "${coreutils}/bin/kill -HUP $MAINPID";
       };
       preStart = ''