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.
Just following the current convention with the underlying TestingZookeeperServer. It throws an exception if restart() is called on an instance that is not in a STOPPED state. Are you suggesting that restart() should work from any state? Or just a LATENT or STOPPED state? Either way, it would require changes to the TestingZookeeperServer, but I don't think that's inherently a problem as the restart() method is only used by the TestingCluster, and I don't think modifying the behaviour would cause issues there.