Replace ConfigurationItem collection with MemoryStorage

Problem/Motivation

In FeaturesManager::getConfigCollection() we return an array of ConfigurationItem objects, read in from existing configuration.

Using this custom approach cuts us off from a lot of the built-in functionality of Drupal's configuration APIs, such as support for multiple configuration collections (and hence multilingual support), use of configuration storages and related events, and so on.

Proposed resolution

Use a MemoryStorage for configuration items.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet