Add statistics that give insight to cached disk image size

It would be useful to be able to look at WiredTiger statistics and get a feel for common disk-image sizes associated with pages in the cache.

This might look like a histogram with counts of page sizes in buckets.

Those statistics would also be interesting per table (collection or index).

Having the information would help us reason about some application behaviors regards cache and memory utilization, and would also give us a tool for checking to ensure a healthy on-disk tree is being generated.