Some test utility refactoring in preparation for elasticity tests.
Another small refactor for test utility simplification.
Stopping a session now immediately removes it from callbacks. Prior to this the stopping of a session would result in callbacks on the session that was being stopped. That behavior is odd. The FIRST thing that should happen when a session is stopped is that it will not longer receive callbacks or be able to participate in the ClusterInfo management. This changes make this the case.