TypedDataManager prototypes should not include the parent context

Problem/Motivation

TypedDataManager creates property prototypes - e.g. a typed data instance of 'field_something' based on the entity/bundle/field name and static caches them. This is so that if multiple entities of the same type are loaded and the same fields are requested, it doesn't have to create the full typed data object every time.

However, this static cache also stores the parent typed data object within the prototype. When the parent is an entity object, this creates a static cache of entities that can never be cleared, making entity LRU caching ineffective. It will only include the entities for the first time a field is seen, but still not good.

Split from #3572625: Calling $entity->getTranslatedField() results in an entity-sized memory leak where I found several other issues with circular references and static caching in the typed data system but this is something that can be done self-contained.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

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