Use a singleton threadpool for kex message handler flushing

sshd-core/src/main/java/org/apache/sshd/common/session/helpers/KeyExchangeMessageHandler.java

@@ -28,7 +28,6 @@
 import java.util.Queue;
 import java.util.concurrent.ConcurrentLinkedQueue;
 import java.util.concurrent.ExecutorService;
-import java.util.concurrent.Executors;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
@@ -75,18 +74,18 @@ public class KeyExchangeMessageHandler {
  // re-acquires the write lock, so normally not many readers (i.e., writePacket() calls) will get a chance to enqueue
  // new packets.
 
- /**
- * We need the flushing thread to have priority over writing threads. So we use a lock that favors writers over
- * readers, and any state updates and the flushing thread are writers, while writePacket() is a reader.
- */
- protected final ReentrantReadWriteLock lock = new ReentrantReadWriteLock(false);
-
  /**
  * An {@link ExecutorService} used to flush the queue asynchronously.
  *
  * @see #flushQueue(DefaultKeyExchangeFuture)
  */
- protected final ExecutorService flushRunner = Executors.newSingleThreadExecutor();
+ protected static ExecutorService flushRunner = ThreadUtils.newCachedThreadPool("kex-flusher");
+
+ /**
+ * We need the flushing thread to have priority over writing threads. So we use a lock that favors writers over
+ * readers, and any state updates and the flushing thread are writers, while writePacket() is a reader.
+ */
+ protected final ReentrantReadWriteLock lock = new ReentrantReadWriteLock(false);
 
  /**
  * The {@link AbstractSession} this {@link KeyExchangeMessageHandler} belongs to.
@@ -209,7 +208,6 @@ public void shutdown() {
  kexFlushedFuture.get());
  });
  items.getValue().setValue(Boolean.valueOf(items.getKey().intValue() == 0));
- flushRunner.shutdownNow();
  }
 
  /**