refactor(model): use internal parallelLimit() instead of async

Re: #8073

lib/helpers/parallelLimit.js

@@ -0,0 +1,51 @@
+'use strict';
+
+module.exports = parallelLimit;
+
+/*!
+ * ignore
+ */
+
+function parallelLimit(fns, limit, callback) {
+  let numInProgress = 0;
+  let numFinished = 0;
+  let error = null;
+
+  if (fns.length === 0) {
+    return callback();
+  }
+
+  for (let i = 0; i < fns.length && i <= limit; ++i) {
+    _start();
+  }
+
+  function _start() {
+    fns[numFinished + numInProgress](_done(numFinished + numInProgress));
+    ++numInProgress;
+  }
+
+  const results = [];
+
+  function _done(index) {
+    return (err, res) => {
+      --numInProgress;
+      ++numFinished;
+
+      if (error != null) {
+        return;
+      }
+      if (err != null) {
+        error = err;
+        return callback(error);
+      }
+
+      results[index] = res;
+
+      if (numFinished === fns.length) {
+        return callback(null, results);
+      } else if (numFinished + numInProgress < fns.length) {
+        _start();
+      }
+    };
+  }
+}

lib/model.js

@@ -38,7 +38,7 @@ const isPathSelectedInclusive = require('./helpers/projection/isPathSelectedIncl
 const get = require('./helpers/get');
 const leanPopulateMap = require('./helpers/populate/leanPopulateMap');
 const modifiedPaths = require('./helpers/update/modifiedPaths');
-const parallelLimit = require('async/parallelLimit');
+const parallelLimit = require('./helpers/parallelLimit');
 const util = require('util');
 const utils = require('./utils');