Expose DefaultThreadFactory.threadGroup to sub-classes

Motivation:

DefaultThreadFactory allows to override the newThread(...) method and so should have access to all fields that are set via the constructor.

Modifications:

Change threadGroup from private to protected visibility.

Result:

Easier to extend DefaultThreadFactory.

common/src/main/java/io/netty/util/concurrent/DefaultThreadFactory.java

@@ -34,7 +34,7 @@ public class DefaultThreadFactory implements ThreadFactory {
     private final String prefix;
     private final boolean daemon;
     private final int priority;
-    private final ThreadGroup threadGroup;
+    protected final ThreadGroup threadGroup;
 
     public DefaultThreadFactory(Class<?> poolType) {
         this(poolType, false, Thread.NORM_PRIORITY);