fix(async): rewrote asyncGenerator in node < 10 syntax

Fixes NODE-1922

lib/async/async_iterator.js

@@ -1,15 +1,33 @@
 'use strict';
 
-async function* asyncIterator() {
-  while (true) {
-    const value = await this.next();
-    if (!value) {
-      await this.close();
-      return;
-    }
+// async function* asyncIterator() {
+//   while (true) {
+//     const value = await this.next();
+//     if (!value) {
+//       await this.close();
+//       return;
+//     }
+
+//     yield value;
+//   }
+// }
 
-    yield value;
-  }
+// TODO: change this to the async generator function above
+function asyncIterator() {
+  const cursor = this;
+
+  return {
+    next: function() {
+      return Promise.resolve()
+        .then(() => cursor.next())
+        .then(value => {
+          if (!value) {
+            return cursor.close().then(() => ({ value, done: true }));
+          }
+          return { value, done: false };
+        });
+    }
+  };
 }
 
 exports.asyncIterator = asyncIterator;