[scraper] updates scapers limiter usage

scraper/lib/promises.js

@@ -0,0 +1,29 @@
+/**
+ * Exxecute promises in sequence one after another.
+ */
+async function sequence(promises) {
+  return promises.reduce((promise, func) =>
+      promise.then(result => func().then(Array.prototype.concat.bind(result))), Promise.resolve([]));
+}
+
+/**
+ * Return first resolved promise as the result.
+ */
+function first(promises) {
+  return Promise.all(promises.map((p) => {
+    // If a request fails, count that as a resolution so it will keep
+    // waiting for other possible successes. If a request succeeds,
+    // treat it as a rejection so Promise.all immediately bails out.
+    return p.then(
+        (val) => Promise.reject(val),
+        (err) => Promise.resolve(err)
+    );
+  })).then(
+      // If '.all' resolved, we've just got an array of errors.
+      (errors) => Promise.reject(errors),
+      // If '.all' rejected, we've got the result we wanted.
+      (val) => Promise.resolve(val)
+  );
+}
+
+module.exports = { sequence, first };