Create a new UserRepository for the code from Drupal\user\UserStorage

Problem/Motivation

The database driver for MongoDB needs to override the class Drupal\Core\Entity\Sql\SqlContentEntityStorage. MongoDB does document/JSON storage instead of relational database storage.
The User entity is overriding that class in Drupal\user\UserStorage. The problem for the database driver for MongoDB is that it then also needs to override the methods in Drupal\user\UserStorage. It also needs to duplicate the code inside Drupal\file\FileStorage too. Duplicating code is wrong.
Another problem is that the code in Drupal\user\UserStorage class is that it is NOT extending the functionality from the class Drupal\Core\Entity\Sql\SqlContentEntityStorage. The extra code can easily being moved to another class.

Proposed resolution

- Create a new Drupal\user\UserRepository class/service and move the code/methods from Drupal\user\UserStorage to the new class/service.
- Deprecate the methods from the class Drupal\user\UserStorage.

Warning: do not change the method Drupal\user\UserStorage::doSaveFieldItems().

Remaining tasks

User interface changes

None

API changes

- New service user_repository with the methods updateLastLoginTimestamp(), updateLastAccessTimestamp() and deleteRoleReferences().
- The methods updateLastLoginTimestamp(), updateLastAccessTimestamp() and deleteRoleReferences() are deprecated.

Data model changes

None

Release notes snippet

None