Cache users by objectID, and clear cache when updated via master key (fixes #1836) (#1844)

* Cache users by objectID, and clear cache when updated via master key

* Go back to caching by session token. Clear out cache by querying _Session when user is modified with Master Key (ew, hopefully that can be improved later)

* Fix issue with user updates from different sessions causing stale reads

* Tests aren't transpiled...

* Still not transpiled

src/Routers/UsersRouter.js

@@ -85,6 +85,7 @@ export class UsersRouter extends ClassesRouter {
         user = results[0];
         return passwordCrypto.compare(req.body.password, user.password);
       }).then((correct) => {
+
         if (!correct) {
           throw new Parse.Error(Parse.Error.OBJECT_NOT_FOUND, 'Invalid username/password.');
         }