Metadata cursor

@michaelcahill, here's a possible implementation of the metadata cursor, ref #659.   If this is what you had in mind, cool, otherwise, point me at what you're thinking and I'll fix it.

One thing if you're tempted to merge this change: I'm getting an error "Unreferenced page: see docs/pages.html for the list", for the page I created, but I don't see how to fix it.