Move the stats to its own contrib extension

The statistics feature is deprecated and disabled by default since XWiki 8.0.

Ideally it should not be embedded anymore in XWiki Standard and move to its own contrib repository, but it's not that easy:

• it needs a custom table, but this is not easy to deal with for contrib extensions
• the hibernate store current has some special handling of statistics that needs to be made less hardcoded
• most of it is still part of xwiki-platform-oldcore and the XWiki class expose publicly some stats related classes or features (like #getSearchEngineRules and #getRefererText)