Unique indexes (#1971)

* Add unique indexing

* Add unique indexing for username/email

* WIP

* Finish unique indexes

* Notes on how to upgrade to 2.3.0 safely

* index on unique-indexes: c454180 Revert "Log objects rather than JSON stringified objects (#1922)"

* reconfigure username/email tests

* Start dealing with test shittyness

* Remove tests for files that we are removing

* most tests passing

* fix failing test

* Make specific server config for tests async

* Fix more tests

* fix more tests

* Fix another test

* fix more tests

* Fix email validation

* move some stuff around

* Destroy server to ensure all connections are gone

* Fix broken cloud code

* Save callback to variable

* no need to delete non existant cloud

* undo

* Fix all tests where connections are left open after server closes.

* Fix issues caused by missing gridstore adapter

* Update guide for 2.3.0 and fix final tests

* use strict

* don't use features that won't work in node 4

* Fix syntax error

* Fix typos

* Add duplicate finding command

* Update 2.3.0.md

src/Adapters/Storage/Mongo/MongoCollection.js

@@ -83,6 +83,18 @@ export default class MongoCollection {
     return this._mongoCollection.deleteMany(query);
   }
 
+  _ensureSparseUniqueIndexInBackground(indexRequest) {
+    return new Promise((resolve, reject) => {
+      this._mongoCollection.ensureIndex(indexRequest, { unique: true, background: true, sparse: true }, (error, indexName) => {
+        if (error) {
+          reject(error);
+        } else {
+          resolve();
+        }
+      });
+    });
+  }
+
   drop() {
     return this._mongoCollection.drop();
   }