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.
I think that's the way it works in practice, but I don't think there's any particular requirement it be the case, and since this is an array of structures, each owned by a separate thread, I figured better safe than sorry. (FTR, we have other structures with 64-bit fields with the same alignment attribute command.)