Try to reduce impact of the notifications counter and notifications display on the instance

See XWIKI-16207 for more details.

In short getting the current user notifications is very expensive and done very often which can put the whole instance down pretty quickly when there is a lot of user and/or a lot of events.

While waiting for a long term refactoring on XWIKI-16207 the idea is to reduce as much as possible the impact of this service on the rest of the XWiki instance by caching the result and reduce the number of threads allowed to gather notifications.