Page loses its checkout reference when child page creates a checkout

[cd250693]

When a parent page is checked out and one of its child pages is checked out, the reference the parent page has to its checkout is lost.
The result is the parent page is locked, and the checkout changes are lost (they can no longer be checked in).

Steps to reproduce:

• Create a parent page and a child page within it.
• Check out the parent page
• Check out the child page within the original parent page
  The original parent page will now be locked and has lost its reference to its checkout.

Plone Version: 5.0.5
plone.app.iteration version: 3.2.0