API controllers save the instance a second time after each change

Problem/Motivation

Each instance method that changes a layout, such as attachToRoot(), setSource() or setThirdPartySettings(), already saves the new revision through Instance::setNewPresent().
The API controllers then save the instance again, in nine places of ApiController and ApiActionsController.
The second save changes nothing, but it costs a full entity save, and it drops the instance from the entity cache once more, so the next load rewrites it.

Measured on a 2690-node layout, the extra save costs 44 to 60 ms per change, plus the cache rewrite; on an article display, 3 to 36 ms.

Proposed resolution

Remove the controller saves that follow an instance method which already saves. Add a kernel test that a change through the API saves the instance once.

Remaining tasks

• Remove the nine redundant saves.
• Kernel test: one save per change, for a library drop, a move, a settings update and a paste.

User interface changes

None.

API changes

None.

Data model changes

None.