KeyValueFactory is swappable, add an interface and fix type-hints

The installer contains a smart hack to temporarily override $settings to use the KeyValueMemoryFactory instead of the default database factory implementation.

This override and temporary adjustment (and rollback) of $settings can be avoided by registering the KeyValueMemoryFactory directly in the service container.

Doing so reveals that the KeyValueFactory service is swappable, but some other services are using type-hints that explicitly ask for the default implementation. Hence, we need to add an interface for the factory service.