Add cache for getTopUsers

See #147

src/utils/createMemoryCache.js

@@ -0,0 +1,42 @@
+module.exports = function createMemoryCache(memoryFn, cacheTimeMs) {
+  // holds the promise results
+  const cache = new Map;
+  // holds the promises that are not fulfilled
+  const promiseMemory = new Map;
+  return function (...args) {
+    // create cacheKey by joining arguments as string
+    const cacheKey = args.join('.');
+    // check if promising is already running
+    if (promiseMemory.has(cacheKey)) {
+      return promiseMemory.get(cacheKey);
+    }
+    else {
+      // check if result is in cache
+      if (cache.has(cacheKey)) {
+        const cacheItem = cache.get(cacheKey);
+        const now = Date.now();
+        // check if cache is valid
+        if (!(cacheItem.cacheTime + cacheTimeMs < now)) {
+          return Promise.resolve(cacheItem.result);
+        }
+      }
+      // create new promise
+      const promise = new Promise(async (resolve, reject) => {
+        resolve((await memoryFn(...args)));
+      });
+      // store promise reference until fulfilled
+      promiseMemory.set(cacheKey, promise);
+      return promise.then(result => {
+        // store promise result in cache
+        cache.set(cacheKey, {
+          result,
+          cacheTime: Date.now(),
+        });
+        // remove fulfilled promise from memory
+        promiseMemory.delete(cacheKey);
+        // return promise result
+        return result;
+      });
+    }
+  };
+};