Expose a metric for how much a tablet needs to be compacted

Now that the maintenance manager is fast at scheduling tasks, I've seen clusters running 1.4 that are churning through compactions at a high rate with seemingly no end in sight. At least it feels like it, but there's no easy way to verify.

I think it would be good to have some measure of how "uncompacted" a tablet is. Todd thinks we could just use the average "height" of what's seen on the "Rowset Layout Diagram" page.