Most popular label macro: enable caching

At this moment, most popular macro does not cache the results. As a result, a page with a "most popular macro" would be slow (30 seconds or even more).

We need a solution which allows to cache data. Cache TTL must depend on the space/site size.

There should be an option to disable this cache via a new system variable:

-Dconfluence.popular-label-macro.disable-cache=true