The tag cloud on large wikis slows down pages like panels and the dashboard

Steps to reproduce:

1. Have a lot of pages with tags.
2. Open the dashboard or the panel administration.

Expected result:

The page loads fast.

Actual result:

The page is slow, because the tag cloud is slow.

This is similar to XWIKI-22976 but the intention here is different: it might not be easy to make the tag cloud faster, but we can make the other pages faster by making the cloud load async by making the tag cloud macro async, which is the fix I propose for this issue.