Add /_stats endpoint

Also, remove ingest_stats worker and use the cache instead.

client/utils/addEvent.js

@@ -0,0 +1,9 @@
+const addEvent = (node, type, handler) => {
+  if (node.addEventListener) {
+    node.addEventListener(type, handler, false)
+  } else if (node.attachEvent) {
+    node.attachEvent('on' + type, handler)
+  }
+}
+
+export default addEvent