Problem/Motivation
While working on #3217531: Deprecate usage of Connection::getDriverClass for some classes, and use standard autoloading instead, we came across the fact that Connection::getDriverClass is also used to resolve overrides to select query extenders. This is not easily solvable with direct autoloading because the to-be-overridden class resides outside of the database driver namespaces; also, while the db driver specific classes are all necessarily extensions of the Core\Database ones, the extenders may be freely added by any module.
Proposed resolution
Try using backend-overrideable services, adding a factory for each extender that will then instantiate extender objects.
Remaining tasks
Needs followup, see #58
User interface changes
None
API changes
Using the full class name as the parameter value in the method Drupal\Core\Database\Query\Select::extend() has been deprecated. Also the method Drupal\Core\Database\Connection::getPagerManager() has been deprecated.
Data model changes
None
Release notes snippet
Select query extenders are now managed through backend-overrideable services. When extending a query, consuming code needs to switch from hardcoding the extension class to calling the extender service with the type of extension required. Contrib and custom database drivers overriding the extenders need to implement their own service. See https://www.drupal.org/node/3218001