Close stats

Add a new statistics_log=(on_close=bool) option to wiredtiger_open.

Not yet ready for review or merge.
I added a new flag field for this, rather than using the __wt_connection_impl flags field for two reasons:
1) The connection_impl field is fairly well used already, and I quite like keeping stats separate
2) The flags here are shared with __wt_cursor_stat, so keeping them separate makes that mapping more obvious.