hydra-evaluator improvements

* The "Jobset" page now shows when evaluations are in progress (rather
  than just pending).

* Restored the ability to do a single evaluation from the command line
  by doing "hydra-evaluator <project> <jobset>".

* Fix some consistency issues between jobset status in PostgreSQL and
  in hydra-evaluator. In particular, "lastCheckedTime" was never
  updated internally.

hydra-module.nix

@@ -345,6 +345,7 @@ in
         environment = env;
         serviceConfig =
           { ExecStart = "@${cfg.package}/bin/hydra-evaluator hydra-evaluator";
+            ExecStopPost = "${cfg.package}/bin/hydra-evaluator --unlock";
             User = "hydra";
             Restart = "always";
             WorkingDirectory = baseDir;