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.
Did forcing 64-bit alignment make a difference?  I thought structures would automatically be 64-bit aligned if they contain 64-bit fields?