Fix RejectedExecutionException when using DnsAddressResolverGroup

Motivation:

AddressResolverGroup adds a listener to the termination future of an
EventExecutor when a new AddressResolver is created. The listener calls
AddressResolver.close() when the EventExecutor is terminated to give the
AddressResolver a chance to release its resources.

When using DnsAddressResolverGroup, the AddressResolver.close() will
eventually trigger DnsNameResolver.close(), which closes its underlying
DatagramChannel.

DatagramChannel.close() (or any Channel.close()) will travel through
pipeline and trigger EventExecutor.execute() because
DnsNameResolver.close() has been invoked from a non-I/O thread.
(NB: A terminationFuture is always notified from the GlobalEventExecutor
thread.)

However, because we are doing this in the listener of the termination
future of the terminated EventLoop we are trying to execute a task upon,
the attempt to close the channel fails due to RejectedExecutionException.

Modifications:

- Do not call Channel.close() in DnsNameResolver.close() if the Channel
  has been closed by EventLoop already

Result:

No more RejectedExecutionException when shutting down an event loop.

resolver-dns/src/main/java/io/netty/resolver/dns/DnsNameResolver.java

@@ -307,7 +307,9 @@ public class DnsNameResolver extends InetNameResolver {
      */
     @Override
     public void close() {
-        ch.close();
+        if (ch.isOpen()) {
+            ch.close();
+        }
     }
 
     @Override