QPID-6208: [Java Broker] Avoid possibility of race condition when setting the permitted node list

* Environment thread and broker configuration thread could race, which had the potential to lead to the
  permittedNodeList being removed.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1637845 13f79535-47bb-0310-9956-ffa450edef68