Documents containing XClass definitions aren't cached anymore causing high database load

Steps to reproduce:

1. Repeatedly access some pages of the wiki
2. Monitor database queries

Expected result:

There are no documents loaded from the database as on repeated access, they are cached.

Actual result:

Lots of documents containing XClass definitions are repeatedly loaded from the database, like XWiki.DocumentSheetBinding

This is because due to a bug in the implementation of XWIKI-22782, cloning the XClass marks the document as dirty and since XWIKI-22892, this means that the cache isn't used anymore.