XWikiDocument#display is a lot slower than it should be

Mostly come from XWIKI-8085.

It's looking everywhere if it can find anything that look like a custom displayer and execute it to see if the result is an empty String. It does that for any field and never remember what it found. Also it redo all that once again for the fields actually having custom displayers.

This is an big issue because of Object.get which is often (wrongly) used to get the value which mean it's used a lot.

For example in 6.2RC1 XWikiDocument#display is called about 50 times and take 30% of the time spend to display an empty page.