fix(index): add useCreateIndex option to avoid deprecation warnings

Re: #6880

lib/index.js

@@ -86,6 +86,7 @@ Mongoose.prototype.STATES = STATES;
  * Currently supported options are:
  * - 'debug': prints the operations mongoose sends to MongoDB to the console
  * - 'bufferCommands': enable/disable mongoose's buffering mechanism for all connections and models
+ * - 'useCreateIndex': false by default. Set to `true` to make Mongoose's default index build use `createIndex()` instead of `ensureIndex()` to avoid deprecation warnings from the MongoDB driver.
  * - 'useFindAndModify': true by default. Set to `false` to make `findOneAndUpdate()` and `findOneAndRemove()` use native `findOneAndUpdate()` rather than `findAndModify()`.
  * - 'cloneSchemas': false by default. Set to `true` to `clone()` all schemas before compiling into a model.
  * - 'applyPluginsToDiscriminators': false by default. Set to true to apply global plugins to discriminator schemas. This typically isn't necessary because plugins are applied to the base schema and discriminators copy all middleware, methods, statics, and properties from the base schema.

lib/model.js

@@ -983,7 +983,11 @@ Model.init = function init(callback) {
   const autoIndex = this.schema.options.autoIndex;
   this.$init = new Promise((resolve, reject) => {
     if (autoIndex || (autoIndex == null && this.db.config.autoIndex)) {
-      this.ensureIndexes({ _automatic: true }, function(error) {
+      const options = {
+        _automatic: true,
+        createIndex: this.base.options.useCreateIndex
+      };
+      this.ensureIndexes(options, function(error) {
         if (error) {
           return reject(error);
         }
@@ -1185,9 +1189,7 @@ Model.ensureIndexes = function ensureIndexes(options, callback) {
 
 /**
  * Similar to `ensureIndexes()`, except for it uses the [`createIndex`](http://mongodb.github.io/node-mongodb-native/2.2/api/Collection.html#createIndex)
- * function. The `ensureIndex()` function checks to see if an index with that
- * name already exists, and, if not, does not attempt to create the index.
- * `createIndex()` bypasses this check.
+ * function.
  *
  * @param {Object} [options] internal options
  * @param {Function} [cb] optional callback