Reduce leaf page size

@michaelcahill: I dropped the default leaf page size to 32KB (since the size applies before block compression, the actual I/O size will be roughly half that?).  If you'd rather go even smaller before considering compression, as I said, I have no objection.

I also changed format to focus on smaller key/value items, we really shouldn't be spending so much time on overflow items.   But, could we add a couple of stress tests that focus on overflow key/value items so they do get tested?

Reference #791.