Outcome: Allow each test/checkpoint run to be configured to switch b/w leader and follower multiple times.

Even though test_checkpoint cannot create checkpoints in follower mode, it still makes sense to:

• in leader mode, do regular test_checkpoint ops, including checkpoints
• switch to follower mode, do regular test_checkpoint ops (no checkpoints)
• switch to leader mode, do regular test_checkpoints ops, including checkpoints
• finish and verify that checkpoints contain expected stuff.

Out of scope: Fixing disagg related bugs that comes as a result of implementing switch mode.