Permanently cache discovered bindings instead of re-executing at XWiki startup

It takes really long to initialize (1mn+). To alleviate for this, it would be nice that the cached data be saved on disk in the permanent directory so that the initi is not done any at each startup. Note that we also probably need a button to refresh the caches and rescan the wiki after an extension is installed (unless there's a listener implemented already that does an automatic scanning when an extension is installed).