Replace InMemoryStorage with MemoryStorage from core

Problem/Motivation

Currently we use an InMemoryStorage. An equivalent (and refined) MemoryStorage was added to core in #3015886: Add an in-memory config storage. #3033200: Use core's MemoryStorage awaits a core 8.7 stable release. Meantime, we can replace our code with the version in core.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet