Cache "users viewing" for each forum and in threads

[beet]

Hello,

Everytime I enable users viewing for my showthread and fordisplay and forumhome, I take a massive CPU hit. I even just enabled it for forumhome, and when the forum gets busy, you can see it the load average go up a few points.

I like the idea of seeing which forums are popular but the only way that can happen is to cache it every 10 mintutes or so.

Thanks to anyone who undertakes this project!