Format timer

@michaelcahill, changes to support timer-limited runs in format, some minor cleanups.

The reason I didn't just push this is that I changed how we're calculating how often to checkpoint and/or close/open session/cursors, and I thought that might be worth a quick look.
Put another way, my first thought for the calculation here would be `(g.c_ops + g.c_threads - 1) / g.c_threads`.  Adding 100 seems pretty arbitrary.