Introduce ReferenceCounted.refCnt()

Motivation:

When debugging netty memory leaks, it's sometimes helpful to
print the object's reference count.

Modifications:

Add `refCnt` methods to set of already exitsting helpers for ref coutned
objects.

Result:

Users will have utility to print object's ref count without much of a
boilerplate.

common/src/main/java/io/netty/util/ReferenceCountUtil.java

@@ -131,6 +131,14 @@ public final class ReferenceCountUtil {
         return msg;
     }
 
+    /**
+     * Returns reference count of a {@link ReferenceCounted} object. If object is not type of
+     * {@link ReferenceCounted}, {@code -1} is returned.
+     */
+    public static int refCnt(Object msg) {
+        return msg instanceof ReferenceCounted ? ((ReferenceCounted) msg).refCnt() : -1;
+    }
+
     /**
      * Releases the objects when the thread that called {@link #releaseLater(Object)} has been terminated.
      */