Missing HTTP Cache purge after copying subtree.

Steps to reproduce

1. Create the following structure:

a) Home/Folder A
b) Home/Folder A/Folder B
c) Home/Folder A/Folder B/Article

2. Copy subtree Folder B into Folder A
3. Copy subtree Folder B into Folder A again
4. Go to location view for Folder A
5. The pagination is broken (see attached screenshot) because childCount in response to GET /api/ezp/v2/content/location/X/Y is out of date.

Steps between 2 and 5 need to be done in time less than default_ttl. Discovered by QA while testing EZP-27759: https://github.com/ezsystems/PlatformUIBundle/pull/893#issuecomment-340418960