Previous tickets added leader and follower mode to test/format. We need a way to test both, and transitions between them. When we are testing using "switch", we'll start in either leader or follower mode, and periodically switch between them.  Leaders can do checkpoints, followers cannot.

When switching from leader to follower (step-down), any data modified since the last checkpoint is disregarded (i.e. gets lost). This is part of the WT contract behavior.  For test/format, it means that at a step-down, we need to lost knowledge of recent changes that have been performed (they care kept in the "snap" data structure).  Normally "snap" is used to re-read data that has been recently written to verify that it is still there.  Since it won't be, the safest course of action (to get successful runs) is to zero out the snap at step-down.  There may be some thread coordination to pull this off.