storage: rework listDir api to be a generator (like) function

src/storage/noop.js

@@ -61,14 +61,13 @@ async function download(apiConfig, backupFilePath) {
     throw new BoxError(BoxError.NOT_IMPLEMENTED, 'Cannot download from noop backend');
 }
 
-function listDir(apiConfig, dir, batchSize, iteratorCallback, callback) {
+async function listDir(apiConfig, dir, batchSize, marker) {
     assert.strictEqual(typeof apiConfig, 'object');
     assert.strictEqual(typeof dir, 'string');
     assert.strictEqual(typeof batchSize, 'number');
-    assert.strictEqual(typeof iteratorCallback, 'function');
-    assert.strictEqual(typeof callback, 'function');
+    assert(typeof marker !== 'undefined');
 
-    callback();
+    return { entries: [], marker: null };
 }
 
 async function copy(apiConfig, oldFilePath, newFilePath, progressCallback) {