Add persistent cache to CachedStorage::findByPrefix

Problem/Motivation

We initially didn't add caching to this because it would need to be invalidated fairly frequently with cache tags.

However, some of these calls can be fairly frequent and aren't cached (enough), specifically LanguageManager::getLanguages().

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet