Motivation:
RoundRobinDnsAddressResolverGroup ultimately opens UDP
ports for DNS resolution. Callers likely expect that
RoundRobinDnsAddressResolverGroup#close() will close those
ports, but that is not currently true (see #9212).
Modifications:
Overrode RoundRobinInetAddressResolver#close() to close
the delegate name resolver, which in turn closes any UDP
ports used for name resolution.
Result:
RoundRobinDnsAddressResolverGroup#close() closes UDP ports
as expected. This fixes#9212.