Chore: make populateSettings middleware use node-cache

lib/middlewares/state.js

@@ -1,7 +1,13 @@
+const NodeCache = require('node-cache')
+const SETTINGS_CACHE_REFRESH = 3600
+
 module.exports = (function () {
   return {
     oldVersionId: 'unset',
-    settingsCache: {},
+    settingsCache: new NodeCache({
+      stdTTL: SETTINGS_CACHE_REFRESH,
+      checkperiod: SETTINGS_CACHE_REFRESH // Clear cache every hour
+    }),
     canLogClockSkewMap: {},
     canGetLastSeenMap: {},
     pids: {},