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.
Why only make this valid if the server had already been stopped? Just for consistency with the internal TestingZookeeperServer behaviour?