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.
Basically, I didn't want 10 threads and 5 operations, so I tossed in an extra 100 operations, I figured it didn't matter what I used (it was totally arbitrary).

I'll tighten it down to a minimum of 1 operation per thread.