Added index size checks for databases and collections.

We've found that index sizes can grow unexpectedly large, especially in fragmented collections. If this goes on too long, mongodb can end up not being able to keep in memory the entire active working set (which for us includes indexes). We've been surprised by sudden slowdowns, so I added these checks. Now we can keep an eye on the collections most prone to this behavior as well as on the entire database in case a new collection starts ballooning.