Add several changes related to testing:

- Add a new plugin that can be used to terminate client connections at specific
  points in plugin processing when an appropriate control is included in the
  request.

- Add a new plugin that can be used to count the number of times that various
  types of plugins are invoked.  Update the startServer and shutdownServer
  methods in TestCaseUtils to ensure that the startup and shutdown plugins are
  invoked at the right times.

- Add test cases for the DirectoryException and InitializationException
  classes.

- Create an OperationTestCase superclass that can be used for testing generic
  methods in the Operation class.  Create an AbandonOperationTestClass subclass
  that tests the core Abandon operation.