This change relates to issue 3567.

In certain scenarios (e.g. distribution) a single client side persistent search may result in multiple back-end persistent searches being launched. The current server design only supported a one to one relationship. This change removes the restriction:

   * persistent search is no longer stored in the search operation

   * client connection operation cancellation now supports having
     multiple PersistentSearch objects per client search

   * PersistentSearch operation cancellation now cancels all related
     PersistentSearches

   * PersistentSearch instances must be enabled before use: doing this
     registers the persistent search with the client connection and
     prevents the underlying search operation from having a result sent.

I have also updated the class' javadoc to include more complete usage information.