docs: private PBKey buildDefaultKey()
      {
          List descriptors = connectionRepository().getAllDescriptor();
          JdbcConnectionDescriptor descriptor;
          PBKey result = null;
          for (Iterator iterator = descriptors.iterator(); iterator.hasNext();)
          {
              descriptor = (JdbcConnectionDescriptor) iterator.next();
              if (descriptor.isDefaultConnection())
              {
                  if(result != null)
                  {
                      log.error("Found additional connection descriptor with enabled 'default-connection' "
                              + descriptor.getPBKey() + ". This is NOT allowed. Will use the first found descriptor " + result
                              + " as default connection");
                  }
                  else
                  {
                      result = descriptor.getPBKey();
                  }
              }
          }
  
          if(result == null)
          {
              log.info("No 'default-connection' attribute set in jdbc-connection-descriptors," +
                      " thus it's currently not possible to use 'defaultPersistenceBroker()' " +
                      " convenience method to lookup PersistenceBroker instances. But it's possible"+
                      " to enable this at runtime using 'setDefaultKey' method.");
          }
          return result;
      }