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.
Do you have a number of operations per thread in mind here?

There used to be this principle that each thread would do ~5 checkpoints per run -- that was kind of weird with 20 threads, and I think doing less checkpoints is probably a good thing.  That said, the new calculation doesn't make it easy for me to figure out what's going on.

Could format just use the checkpoint server thread?  I don't really expect many applications to have threads that do cursor operations interspersed with checkpoints -- I'm not saying that it isn't worth testing, but we have a simple spin lock that prevents checkpoints from racing...