Unify Hydra's NixOS module with the one used for hydra.nixos.org

In particular, the queue runner and web server now run under different
UIDs.

src/hydra-queue-runner/hydra-queue-runner.cc

@@ -1568,7 +1568,9 @@ void State::notificationSender()
 
 std::shared_ptr<PathLocks> State::acquireGlobalLock()
 {
-    Path lockPath = hydraData + "/queue-runner";
+    Path lockPath = hydraData + "/queue-runner/lock";
+
+    createDirs(dirOf(lockPath));
 
     auto lock = std::make_shared<PathLocks>();
     if (!lock->lockPaths(PathSet({lockPath}), "", false)) return 0;

src/lib/Hydra.pm

@@ -55,7 +55,7 @@ __PACKAGE__->config(
     },
     'Plugin::Session' => {
         expires => 3600 * 24 * 7,
-        storage => ($ENV{'HYDRA_SERVER_DATA'} // Hydra::Model::DB::getHydraPath) . "/session_data",
+        storage => Hydra::Model::DB::getHydraPath . "/www/session_data",
         unlink_on_exit => 0
     },
     'Plugin::AccessLog' => {