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.
Good point, yes it should be passed into the other constructor, will fix this up now.

The start flag is not currently used in any of the unit tests, but I think that it's useful to be able to create a server without starting it. There are cases where you want to start with a server that is not started.