Add /_stats endpoint

Also, remove ingest_stats worker and use the cache instead.

client/utils/removeEvent.js

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