XWikiDocument#isFromCache is not reliable

It's set to true only if the document was already in the cache when it was requested to the XWikiCacheStore, but:

• it's not set to true when the document is loaded from the DB and inserted in the cache
• it's still true if the document is removed from the cache

All that is making it totally unusable.

It would make a lot more sense for XWikiDocument#setFromCache to be triggered from CacheEntryListener#cacheEntryAdded and CacheEntryListener#cacheEntryRemoved.