The list of always used extensions on a wiki is not properly updated when a doc is saved with programming rights

Skin extensions that have use = "always on this wiki" need programming rights in order to be actually used.

The following scenario used to work and it doesn't anymore:

• Import a page with a ssx with use "always on this wiki", with an author which doesn't have programming rights (so the extension will not be pulled)
• Save the page with a user that has programming rights, in order to make it work. Make sure to not change anything at all in the document, just edit and save without touching it
• The ssx will still not be pulled as always used extension until the next restart of the wiki.

This is caused by a cache for the always used extensions per wiki, which is no longer properly updated when a document changes only author, since the modifications for XWIKI-6906.