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.
I agree, I was surprised that restart was really just 'start if you've been stopped before'. It doesn't look like any test cases are using this code at the moment, so I'll modify the restart() to do a stop() if it's running, followed by a start().