Increase default documents cache size

Various XE pages like the home page and the search page often need to manipulate more than 100 documents (many classes, pages for which to display title and links, current user and anything related, etc.) which mean that they end up reloading the same documents at each request.

We should probably increase the default cache size to something like 500 (XE contains 425 documents in 7.0). Most documents are small in term of memory so it's not going to make XWiki takes so much more memory but loading a document is always a slow process especially documents with objects.

Note that it was a lot worst before XWIKI-12046 (it used to reload something like 80 documents and now it's just 3 or 4 in my tests) but 100 is still not enough it seems.