Reference checker should only read relay descriptors

I just noticed that ReferenceChecker reads bridge descriptors in my local test instance. Looks like we changed paths in this commit to include all recent descriptors, not just recent relay descriptors:

-    new ReferenceChecker(new File("recent/relay-descriptors"),
-        new File("stats/references"),
-        new File("stats/references-history")).check();
+    new ReferenceChecker(
+        recentPath.toFile(),
+        new File(config.getPath(Key.StatsPath).toFile(), "references"),
+        new File(config.getPath(Key.StatsPath).toFile(), "references-history")).check();

We should probably change that back to avoid reading descriptors we don't care about anyway.