Avoid expensive operations while client waits for response

Problem/Motivation

When an entity is inserted, updated or deleted, all operations related to links are performed inside the request. They can be expensive and there's no good reason the links generation should be performed while the client waits for the response. We don't need those links instantly. Moreover, creating entities inside the transaction (of node save, for instance) leads potentially to complications.

Proposed resolution

Move the operations in after the response has been sent to the client, in Kernel terminate phase.

Remaining tasks

None.

User interface changes

None.

API changes

Two services to implement DestructableInterface.

Data model changes

None.