CURATOR-114 - Modified the TestingServer to expose the restart() method

Modified the TestingServer to expose the restart() method on the underlying TestingZooKeeperServer. Modified all unit tests that were previously using the stop() and then recreate using existing temporary directory and port approach for restarting the server, so that they now just call the restart() method.
As a user, it would make sense for restart to work from any state. My mental model is akin to Unix services where restart is simply "stop ; start"