New untyped stats API

[cberner]

Fixes #398

[casey]

Nice, that's great. This PR uses the new API. The only issue I see is that database stats are only available on write transactions, whereas open_untyped_table is only available on write transactions, so in order to get all stats, you need to open a read transaction and a write transaction.

[cberner]

ya, I was thinking about adding open_untyped_table() to write transactions too. But then I couldn't decide whether it should return a ReadOnlyUntypedTable or an UntypedTable that might in the future have mutation methods (clear() would be an option).

[casey]

Seems reasonable to me. Thanks for adding this!