Tags block should cache the tags cloud SQL results

We are experiencing performance issues with the Tags block on moodle.org where we use a single tags collection (users' interests) with a few tens of thousands tags and tag instances.

The block calls core_tag_collection::get_tag_cloud() on every single page load and generating the tags cloud take a significant time (we suspect due to sorting involved).

This represents a potential surface for a DoS attack on bigger sites with many tags using the block.

It is apparent that the block could cache the results for a while instead of populating the same aggregated cloud over and over again.