(7.1.0) Avoid inefficient iteration over Map

On few places in Artemis code, inefficient iteration over Map object is present.
Example

  Map<Thread, StackTraceElement[]> threadMap = Thread.getAllStackTraces();
         for (Thread thread : threadMap.keySet()) {
           StackTraceElement[] stack = threadMap.get(thread);
           ....

To make it more efficient, we should use Map.Entry instead of getting keySet and than finding corresponding value.
Places where this could be changed

• ScaleDownHandler#scaleDownDuplicateIDs
• PostOfficeJournalLoader#recoverPendingPageCounters
• ActiveMQTestBase#tearDown