Use LRU cache as default mechanism for caching in memory (#3979)

* Use LRU cache as default mechanism for caching in memory

* Return null as what’s expected

src/defaults.js

@@ -32,6 +32,8 @@ export default {
   expireInactiveSessions: true,
   revokeSessionOnPasswordReset: true,
   schemaCacheTTL: 5000, // in ms
+  cacheTTL: 5000,
+  cacheMaxSize: 10000,
   userSensitiveFields: ['email'],
   objectIdSize: 10
 }