Not existing document should not be counted in the document cache size

When displaying a skin and a document with lots of fields displays in it, we ask for tons of unexisting documents (in case some skin template or some displayer is overwritten for example) at each request meaning that at each request we fill the cache and potentially ditch lots of real documents from the cache that we will have to load again later.

Not existing document should probably not be stored in the document cache and instead just rely on exist cache (meaning directly return a new XWikiDocument when finding an explicit false in exist cache without putting it in he document cache).

A good example of that is to search for "*" with standard config. This asks for more that 100 document most of which never exist and since the maximum cache size is 100 it ends up loading about 30 documents every time.