Store configuration in a MemoryStorage

Problem/Motivation

Currently we store configuration data in a custom class, ConfigurationItem, that is limited to a single configuration collection. While we could consider adding in collection support, doing so would require replicating much of the functionality covered in core's StorageInterface.

Proposed resolution

Refactor to instead extend MemoryStorage, added to core and slated for inclusion in the 8.7 branch, #3015886: Add an in-memory config storage. We could add the class directly to Features in the interim until it's available in a stable core release.

This is a major change. It would break backward compatibility and therefore requires a new branch, 8.x-4.x.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet