DnsNameResolverContext reuse of DnsServerAddressStream without duplicate

Motivation:
DnsServerAddressStream provides an iterator like interface but maybe expected to start at a specific point upon each new usage. If a DnsServerAddressStream is re-used in multiple independent iterations the order of iteration maybe incorrect. DnsNameResolverContext has a fallback DnsServerAddressStream reference if the cache doesn't contain a hit, but it is shared across multiple independent iterations. This may lead to undesirable DNS query order.

Modifications:
- DnsNameResolverContext#getNameServers should duplicate the default DnsServerAddressStream

Result:
Consistent iteration over the default DnsServerAddressStream in DnsNameResolverContext.

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

@@ -750,7 +750,7 @@ abstract class DnsNameResolverContext<T> {
 
     private DnsServerAddressStream getNameServers(String hostname) {
         DnsServerAddressStream stream = getNameServersFromCache(hostname);
-        return stream == null ? nameServerAddrs : stream;
+        return stream == null ? nameServerAddrs.duplicate() : stream;
     }
 
     private void followCname(String cname, final DnsQueryLifecycleObserver queryLifecycleObserver, Promise<T> promise) {