Too many DB requests to extract the wiki from request URLs

The consequence is lowered performances.

For example we you call http://localhost:8080/xwiki/bin/view/Main/WebHome the folowing calls are made:

• XWiki.getXWiki()
• XWiki.getRequestWikiName()
• XWiki.findWikiServer()
• Query Manager call on the DB to locate a descriptor page

The problem is that when no descriptor is found (which happens on a default install), the code does:

this.virtualWikiMap.set(host, wikiName);

And since wikiName is null, the next time findWikiServer is called the following condition evaluate to true and the cycle repeats:

DocumentReference wikiName = this.virtualWikiMap.get(host);
if (wikiName == null) {
...

The solution is to save the fact that there's no wiki descriptor for "localhost" and have some EventListener update that cache if a descriptor is added.