[META] Decouple entity specific methods out of storage handlers

Problem/Motivation

As example TermStorageInterface & CommentStorageInterface are overloaded with "implementation details" methods that should be moved to separate "repository" services like SearchPageRepository & BlockRepository

Proposed resolution

Implement new services & deprecate old methods before 9.x

Remaining tasks

discus, agree, file patches & change records

User interface changes

no

API changes

only introduce new services & deprecate old ones, TBD

Data model changes

no