- It was possible to delete a Ndb_cluster_connection instance while there
  was still instances of Ndb using the deleted Ndb_cluster_connection.
  Since the Ndb instances are created using a reference to the Ndb_cluster_connection
  and thus have pointers into the TransporterFacade and ClusterMgr owned by
  said Ndb_cluster_connection this may lead to usage of NULL pointer's or usage of
  deleted instances(via pointers that point into Ndb_cluster_connection members).
- Fix by waiting  in Ndb_cluster_connection destructor for all related Ndb objects to be released
- Add test case