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/Adapters/Cache/InMemoryCacheAdapter.js

@@ -1,9 +1,9 @@
-import {InMemoryCache} from './InMemoryCache';
+import {LRUCache} from './LRUCache';
 
 export class InMemoryCacheAdapter {
 
   constructor(ctx) {
-    this.cache = new InMemoryCache(ctx)
+    this.cache = new LRUCache(ctx)
   }
 
   get(key) {

src/Adapters/Cache/LRUCache.js

@@ -0,0 +1,33 @@
+import LRU from 'lru-cache';
+import defaults  from '../../defaults';
+
+export class LRUCache {
+  constructor({
+    ttl = defaults.cacheTTL,
+    maxSize = defaults.cacheMaxSize,
+  }) {
+    this.cache = new LRU({
+      max: maxSize,
+      maxAge: ttl
+    });
+  }
+
+  get(key) {
+    return this.cache.get(key) || null;
+  }
+
+  put(key, value, ttl = this.ttl) {
+    this.cache.set(key, value, ttl);
+  }
+
+  del(key) {
+    this.cache.del(key);
+  }
+
+  clear() {
+    this.cache.reset();
+  }
+
+}
+
+export default LRUCache;