kernel: fix static shutDown flag in ThreadPool causing test isolation failures

ThreadPool.shutDown was declared static, so calling safeShutDown() on any
instance permanently poisoned all future instances in the same JVM. In
TCPEchoRawXMLTest, tearDown() after the first test set the flag, then each
subsequent test's setUp() failed with "Thread pool is shut down" (and left
a leaked ServerSocket on port 5555, causing "Address already in use" for the
remaining tests).

Fix: change to an instance field so each ThreadPool tracks its own lifecycle.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: robertlazarski

modules/kernel/src/org/apache/axis2/util/threadpool/ThreadPool.java

@@ -43,7 +43,7 @@
 public class ThreadPool implements ThreadFactory {
     private static final Log log = LogFactory.getLog(ThreadPool.class);
     protected static long SLEEP_INTERVAL = 1000;
-    private static boolean shutDown;
+    private boolean shutDown;
     protected ThreadPoolExecutor executor;
 
     //integers that define the pool size, with the default values set.