Deleting an entity with revisions and file/image field does not release file usage of non-default revisions, causing files to linger

Problem/Motivation

Images/files uploaded to a file/image fields on entities with multiple revisions are never deleted, even when the entity is deleted. Because the file usage counts for the revisions is never released.

Steps to reproduce:

1. create an article node with an image A uploaded to its image field
2. verify that the uploaded file A has its status column in the file_managed table set to 1,
   and it has 1 usage in the file_usage table
3. edit the node, check the "Create new revision" checkbox, remove the original image A and upload a new image B, save
4. verify that the uploaded file B has its status column in the file_managed table set to 1,
   and it has 1 usage in the file_usage table. Verify that the rows for file A are unchanged in both tables.
5. delete the article
6. verify that the uploaded file B has its status column in the file_managed table set to 0, and it has no row anymore in the file_usage table
7. • Expected result: file A has its status column in the file_managed table set to 0, and it has no row anymore in the file_usage table
   • Actual result: file A has its status column in the file_managed table still set to 1, and it still has n1 usage in the file_usage table
8. run cron 6 hours later (this is the default value of the temporary_maximum_age setting in system.file)
9. verify that uploaded file B is now removed (because usage = 0)
10. • expected result: the uploaded file A is still present (because usage > 0)
    • actual result: the uploaded file A is absent (because usage = 0)

Proposed resolution

TBD, see #126 for an summary of comments #1–#125.

Remaining tasks

TBD

User interface changes

None.

API changes

TBD

Data model changes

TBD